A hadith ( /ˈhædɪθ/  or /hɑːˈdiːθ/ ;  Arabic: حديث ḥadīth , plural: ahadith , أحاديث , ʼaḥādīth ) is one of various reports describing the words, actions, or habits of the Islamic prophet Muhammad. The term comes from the Arabic language and means a "report", "account" or "narrative". Unlike the Qur'an, which is the same literary work recognized by all Muslims, the ahadith are not one single same collection.
The ahadith refers to different hadith collections, and different branches of Islam ( Sunni, Shia, Ibadi) consult different collections of hadith, while the relatively small sect of Quranists reject the authority of any of the hadith collections altogether.
Just as the minority Quranists are not a single community, believers of the ahadith, or hadithists, are also not a single community. Hadithists simply share the feature that, in addition to Quran, they incorporate belief and practice of ahadith— though not necessarily the same hadith collection.
Among most hadithists, the importance of ahadith is secondary to Qur'an given that, at least in theory, an Islamic conflict of laws doctrine holds Qur'anic supremacy above ahadith in developing Islamic jurisprudence. Despite this, however, a minority of hadithists have historically placed ahadith at a par with the Qur'an. A smaller minority yet have even upheld ahadith that contradict the Qur'an, thereby in practice placing ahadith above Qur'an, claiming that contradicting ahadith abrogate those parts of the Qur'an with which they conflict.
The hadith literature is based on spoken reports that were in circulation in society after the death of Muhammad. Unlike the Qur'an the hadiths were not quickly and concisely compiled during and immediately after Muhammad's life. Hadith were evaluated and gathered into large collections during the 8th and 9th centuries, generations after the death of Muhammad, after the end of the era of the "rightful" Rashidun Caliphate, over 1,000 km (620 mi) from where Muhammad lived.
Hadith are regarded by hadithists as important tools for understanding the Quran and commentaries ( tafsir ) written on it. Some important elements, which are today taken to be a long-held part of normative traditional Islamic practice and belief, for example, the detailed ritual practice of the five salat (obligatory Islamic prayers), are in fact not mentioned in the Qur'an at all, but are derived solely from the hadith. Hadithists, therefore, maintain that the ahadith are a necessary requirement for the true and proper practice of Islam, as it gives Muslims the nuanced details of Islamic practice and belief in areas where the Qur'an is silent. Quranists, on the contrary, hold the critical view on hadith that anything on which the Qur'an is silent is deliberate because Allah did not hold its detail to be of consequence, and in the case of ahadith that contradict the Qur'an, more so should those ahadith be forcefully rejected outright as a corruption of Islam.
In the classical example of salat (obligatory Islamic prayers), where salat is commanded in the Qur'an, all Muslims agree that salat is an obligatory part of Islamic religious practice. Divergence among Muslims arises, therefore, in how salat is performed. According to hadithists, the details and instructions of how to correctly perform salat, so as to, in their view, "validly" fulfill the Qur'anic commmand of performing salat, can only be found in the ahadith. Despite this, salat is nonetheless performed differently by different hadithist Islamic sects, depending on which hadith collection each hadithist sect relies upon. Quranists, for their part, leave the detail of salat to be a matter between each individual Muslim and Allah, with salat performance done to each Muslim's own individual understanding, interpretation and need. In the Quranists' view, as the Qur'an is deliberately silent on the details of salat, Allah did not hold its detail to be of consequence, so correctly performed salat lies not in any supposed correct details of the performance of prayer, but on a correct intention to perform the prayers, valid however it may be individually performed.
A hadith has two parts in its classic form, the chain of narrators who have transmitted the report (the isnad ), and the main text of the report (the matn ).   Individual hadith are classified by Muslim clerics and jurists as sahih ("authentic"), hasan ("good") or da'if ("weak").  However, different groups and different scholars may classify a hadith differently.
In Arabic, the noun ḥadīth ( Arabic: حديث ḥadīth IPA: [ħaˈdiːθ] ) means "report", "account", or "narrative".   Its Arabic plural is ʾaḥādīth (أحاديث) ( IPA: [ʔaħaːˈdiːθ] ). Hadith also refers to the speech of a person. 
In Islamic terminology, according to Juan Campo, the term hadith refers to reports of statements or actions of Muhammad, or of his tacit approval or criticism of something said or done in his presence,  though some sources ( Khaled Abou El Fadl) limit hadith to verbal reports, with the deeds of Muhammad and reports about his companions being part of the Sunnah not hadith. 
Classical hadith specialist Ibn Hajar al-Asqalani says that the intended meaning of hadith in religious tradition is something attributed to Muhammad but that is not found in the Quran.
Other associated words possess similar meanings including: khabar (news, information) often refers to reports about Muhammad, but sometimes refers to traditions about his companions and their successors from the following generation ; conversely, athar (trace, vestige) usually refers to traditions about the companions and successors, though sometimes connotes traditions about Muhammad. The word sunnah (custom) is also used in reference to a normative custom of Muhammad or the early Muslim community. 
The two major aspects of a hadith are the text of the report (the matn ), which contains the actual narrative, and the chain of narrators (the isnad ), which documents the route by which the report has been transmitted.  The isnad was an effort to document that a hadith had actually come from Muhammad, and Muslim scholars from the eighth century until today have never ceased repeating the mantra "The isnad is part of the religion - if not for the isnad, whoever wanted could say whatever they wanted." The isnad means literally 'support', and it is so named due to the reliance of the hadith specialists upon it in determining the authenticity or weakness of a hadith.  The isnad consists of a chronological list of the narrators, each mentioning the one from whom they heard the hadith, until mentioning the originator of the matn along with the matn itself.
The first people to hear hadith were the companions who preserved it and then conveyed it to those after them. Then the generation following them received it, thus conveying it to those after them and so on. So a companion would say, "I heard the Prophet say such and such." The Follower would then say, "I heard a companion say, 'I heard the Prophet.'" The one after him would then say, "I heard someone say, 'I heard a Companion say, 'I heard the Prophet... " and so on. 
Different branches of Islam refer to different collections of hadith, though the same incident may be found in hadith in different collections:
- In the Sunni branch of Islam, the canonical hadith collections are the six books , of which Sahih al-Bukhari and Sahih Muslim generally have the highest status. The other books of hadith are Sunan Abu Dawood, Jami` at-Tirmidhi, Al-Sunan al-Sughra and Sunan ibn Majah. However the Malikis, one of the four Sunni "schools of thought" ( madhhabs ), traditionally reject Sunan ibn Majah and assert the canonical status of Muwatta Imam Malik.
- In the Twelver Shi'a branch of Islam, the canonical hadith collections are the Four Books : Kitab al-Kafi, Man la yahduruhu al-Faqih, Tahdhib al-Ahkam, and Al-Istibsar.
- In the Ibadi branch of Islam, the main canonical collection is the Tartib al-Musnad. This is an expansion of the earlier Jami Sahih collection, which retains canonical status in its own right.
- The Ismaili shia sects use the Daim al-Islam as hadith collections.
- The Ahmadiyya sect generally rely on the Sunni canons.
- Some minor groups, collectively known as Quranists, reject the authority of the hadith collections altogether.
Impact on Islam
The hadith had a profound and controversial influence on moulding the commentaries ( tafsir ) of the Quran. The earliest commentary of the Quran known as Tafsir Ibn Abbas is sometimes attributed to the companion Ibn Abbas, but this is rejected by scholars.
The hadith were used in forming the basis of Sharia (the religious law system forming part of the Islamic tradition), and the hadith are at the root of why there is no single Sharia system, but rather a collection of parallel Sharia systems within Islam.
Much of early Islamic history available today is also based on the hadith and is challenged for lack of basis in primary source material, as well as internal contradictions of the secondary material available.
History, tradition and usage
Traditions of the life of Muhammad and the early history of Islam were passed down mostly orally for more than a hundred years after Muhammad's death in AD 632. Muslim historians say that Caliph Uthman ibn Affan (the third khalifa (caliph) of the Rashidun Empire, or third successor of Muhammad, who had formerly been Muhammad's secretary), is generally believed to urge Muslims to record the hadith just as Muhammad suggested to some of his followers to write down his words and actions.  
Uthman's labours were cut short by his assassination, at the hands of aggrieved soldiers, in 656. No sources survive directly from this period so we are dependent on what later writers tell us about this period. 
According to British historian of Arab world Alfred Guillaume, it is "certain" that "several small collections" of hadith were "assembled in Umayyad times."
In 851 the rationalist Mu`tazila school of thought fell from favor in the Abbasid Caliphate. The Mu`tazila, for whom the "judge of truth ... was human reason,"  had clashed with traditionists who looked to the literal meaning of the Quran and hadith for truth. While the Quran had been officially compiled and approved, hadiths had not. One result was the number of hadiths began "multiplying in suspiciously direct correlation to their utility" to the quoter of the hadith ( Traditionists quoted hadith warning against listening to human opinion instead of Sharia; Hanafites quoted a hadith stating that "In my community there will rise a man called Abu Hanifa [the Hanafite founder] who will be its guiding light". In fact one agreed upon hadith warned that, "There will be forgers, liars who will bring you hadiths which neither you nor your forefathers have heard, Beware of them." In addition the number of hadith grew enormously. While Malik ibn Anas had attributed just 1720 statements or deeds to the Muhammad, it was no longer unusual to find people who had collected a hundred times that number of hadith.
Faced with a huge corpus of miscellaneous traditions supported differing views on a variety of controversial matters—some of them flatly contradicting each other—Islamic scholars of the Abbasid sought to authenticate hadith. Scholars had to decide which hadith were to be trusted as authentic and which had been invented for political or theological purposes. To do this, they used a number of techniques which Muslims now call the science of hadith. 
Shia and Sunni textual traditions
Sunni and Shia hadith collections differ because scholars from the two traditions differ as to the reliability of the narrators and transmitters. Narrators who took the side of Abu Bakr and Umar rather than Ali, in the disputes over leadership that followed the death of Muhammad, are seen as unreliable by the Shia; narrations sourced to Ali and the family of Muhammad, and to their supporters, are preferred. Sunni scholars put trust in narrators, such as Aisha, whom Shia reject. Differences in hadith collections have contributed to differences in worship practices and shari'a law and have hardened the dividing line between the two traditions.
Extent and nature in the Sunni tradition
In the Sunni tradition, the number of such texts is ten thousand plus or minus a few thousand.  But if, say, ten companions record a text reporting a single incident in the life of the prophet, hadith scholars can count this as ten hadiths. So Musnad Ahmad, for example, has over 30,000 hadiths—but this count includes texts that are repeated in order to record slight variations within the text or within the chains of narrations. Identifying the narrators of the various texts, comparing their narrations of the same texts to identify both the soundest reporting of a text and the reporters who are most sound in their reporting occupied experts of hadith throughout the 2nd century. In the 3rd century of Islam (from 225/840 to about 275/889),  hadith experts composed brief works recording a selection of about two- to five-thousand such texts which they felt to have been most soundly documented or most widely referred to in the Muslim scholarly community.  The 4th and 5th century saw these six works being commented on quite widely. This auxiliary literature has contributed to making their study the place of departure for any serious study of hadith. In addition, Bukhari and Muslim in particular, claimed that they were collecting only the soundest of sound hadiths. These later scholars tested their claims and agreed to them, so that today, they are considered the most reliable collections of hadith. Toward the end of the 5th century, Ibn al-Qaisarani formally standardized the Sunni canon into six pivotal works, a delineation which remains to this day.
Over the centuries, several different categories of collections came into existence. Some are more general, like the muṣannaf , the muʿjam , and the jāmiʿ , and some more specific, either characterized by the topics treated, like the sunan (restricted to legal-liturgical traditions), or by its composition, like the arbaʿīniyyāt (collections of forty hadiths).
Extent and nature in the Shia tradition
Shi'a Muslims do not use the six major hadith collections followed by the Sunni, as they do not trust many of the Sunni narrators and transmitters. They have their own extensive hadith literature. The best-known hadith collections are The Four Books, which were compiled by three authors who are known as the 'Three Muhammads'. The Four Books are: Kitab al-Kafi by Muhammad ibn Ya'qub al-Kulayni al-Razi (329 AH), Man la yahduruhu al-Faqih by Muhammad ibn Babuya and Al-Tahdhib and Al-Istibsar both by Shaykh Muhammad Tusi. Shi'a clerics also make use of extensive collections and commentaries by later authors.
Unlike Sunnis, Shia do not consider any of their hadith collections to be sahih (authentic) in their entirety. Therefore, every individual hadith in a specific collection must be investigated separately to determine its authenticity.
The mainstream sects consider hadith to be essential supplements to, and clarifications of, the Quran, Islam's holy book, as well as for clarifying issues pertaining to Islamic jurisprudence. Ibn al-Salah, a hadith specialist, described the relationship between hadith and other aspect of the religion by saying: "It is the science most pervasive in respect to the other sciences in their various branches, in particular to jurisprudence being the most important of them." "The intended meaning of 'other sciences' here are those pertaining to religion," explains Ibn Hajar al-Asqalani, "Quranic exegesis, hadith, and jurisprudence. The science of hadith became the most pervasive due to the need displayed by each of these three sciences. The need hadith has of its science is apparent. As for Quranic exegesis, then the preferred manner of explaining the speech of God is by means of what has been accepted as a statement of Muhammad. The one looking to this is in need of distinguishing the acceptable from the unacceptable. Regarding jurisprudence, then the jurist is in need of citing as an evidence the acceptable to the exception of the later, something only possible utilizing the science of hadith."
Hadith studies use a number of methods of evaluation developed by early Muslim scholars in determining the veracity of reports attributed to Muhammad. This is achieved by analyzing the text of the report, the scale of the report's transmission, the routes through which the report was transmitted, and the individual narrators involved in its transmission. On the basis of these criteria, various classifications were devised for hadith. The earliest comprehensive work in hadith studies was Abu Muhammad al-Ramahurmuzi's al-Muhaddith al-Fasil , while another significant work was al-Hakim al-Naysaburi's Ma‘rifat ‘ulum al-hadith . Ibn al-Salah's ʻUlum al-hadith is considered the standard classical reference on hadith studies. 
Terminology: admissible and inadmissible hadiths
By means of hadith terminology, hadith are categorized as ṣaḥīḥ (sound, authentic), ḍaʿīf (weak), or mawḍūʿ (fabricated). Other classifications used also include: ḥasan (good), which refers to an otherwise ṣaḥīḥ report suffering from minor deficiency, or a weak report strengthened due to numerous other corroborating reports; and munkar (denounced) which is a report that is rejected due to the presence of an unreliable transmitter contradicting another more reliable narrator. Both sahīh and hasan reports are considered acceptable for usage in Islamic legal discourse. Classifications of hadith may also be based upon the scale of transmission. Reports that pass through many reliable transmitters at each point in the isnad up until their collection and transcription are known as mutawātir . These reports are considered the most authoritative as they pass through so many different routes that collusion between all of the transmitters becomes an impossibility. Reports not meeting this standard are known as aahad , and are of several different types. 
Some hadith are also called hadith qudsi (sacred hadith), like Ziyarat Ashura. It is a sub-category of hadith which some Muslims regard as the words of God ( Arabic: Allah). According to as-Sayyid ash-Sharif al-Jurjani, the hadith qudsi differ from the Quran in that the former are "expressed in Muhammad's words", whereas the latter are the " direct words of God ". However, note that a hadith qudsi is not necessarily sahih , it can also be da‘if or even mawdu‘ . 
An example of a hadith qudsi is the hadith of Abu Hurairah who said that Muhammad said:
When God decreed the Creation He pledged Himself by writing in His book which is laid down with Him: My mercy prevails over My wrath.
Another area of focus in the study of hadith is biographical analysis ( ‘ilm al-rijāl , lit. "science of people"), in which details about the transmitter are scrutinized. This includes analyzing their date and place of birth; familial connections; teachers and students; religiosity; moral behaviour; literary output; their travels; as well as their date of death. Based upon these criteria, the reliability ( thiqāt ) of the transmitter is assessed. Also determined is whether the individual was actually able to transmit the report, which is deduced from their contemporaneity and geographical proximity with the other transmitters in the chain. Examples of biographical dictionaries include: Abd al-Ghani al-Maqdisi's Al-Kamal fi Asma' al-Rijal , Ibn Hajar al-Asqalani's Tahdhīb al-Tahdhīb and al-Dhahabi's Tadhkirat al-huffaz .
The major points of intra-Muslim criticism of the Hadith literature is based in questions regarding its authenticity. However, Muslim criticism of ahadith is also based on theological and philosophical Islamic grounds of argument and critique.
Muslim scholars have a long history of questioning the Hadith literature throughout Islamic history. Western academics also became active in the field later on.