Abu-Musa Abd-Allah ibn Qays al-Ash'ari , better known as Abu Musa al-Ashari ( Arabic : أبو موسى الأشعري ) (d.ca. 662 or 672) was a companion of Muhammad and an important figure in early Islamic history. He was at various times governor of Basra and Kufa and was involved in the early Muslim conquests of Persia .
Abu Musa came originally from Zabid , region of Yemen , where his tribe, the Ashar, lived in the pre-Islamic period. He accepted Islam at Mecca prior to the hijra and returned to his native Yemen to propagate the faith. There was no news of him for more than a decade until following the conquest of Khaybar in 628 when he came to Muhammad in Medina with more than fifty converts from Yemen including his two brothers Abu Ruhm and Abu Burdah.
Following the conquest of Mecca in 629, Abu Musa was named among those sent by Muhammad on the expedition to Awtas. Two years later he was appointed as one of the governors over Yemen, where he remained until the caliphate of Abu Bakr , whom he joined in fighting the local leader of the ridda (lit. apostasy) movement.
During Muhammad's era
He was present during the Expedition of Dhat al-Riqa . Some scholars claim, the expedition took place in Nejd (a large area of tableland in the Arabian Peninsula) in Rabi‘ Ath-Thani or Jumada Al-Ula, 4 A.H (or beginning of 5AH). They substantiate their claim by saying that it was strategically necessary to carry out this campaign in order to quell the rebellious bedouins in order to meet the exigencies of the agreed upon encounter with the polytheists, i.e. minor Badr Battle in Sha‘ban, 4 A.H. Muhammed received the news that certain tribes of Banu Ghatafan were assembling at Dhat al-Riqa with suspicious purposes.
Muhammad proceeded towards Nejd at the head of 400 or 700 men, after he had mandated Abu Dhar - in the Ummayad version, the Ummayad chief who killed Abu Dhar is given this honor: Uthman bin Affan - to dispose the affairs of Madinah during his absence. The Muslim fighters penetrated deep into their land until they reached a spot called Nakhlah where they came across some bedouins of Ghatfan.  
The most authentic opinion according to "Saifur Rahman al Mubararakpuri", however, is that Dhat Ar-Riqa‘ campaign took place after the fall of Khaibar (and not as part of the Invasion of Nejd). This is supported by the fact that Abu Hurairah and Abu Musa Ashaari witnessed the battle. Abu Hurairah embraced Islam only some days before Khaibar, and Abu Musa Al-Ash‘ari came back from Abyssinia ( Ethiopia ) and joined Muhammad at Khaibar. The rules relating to the prayer of fear which Muhammad observed at Dhat Ar-Riqa‘ campaign, were revealed at the Asfan Invasion and this scholars say, took place after Al-Khandaq (the Battle of the Trench ). 
After the Caliphate of Abu Bakr
The appointments of Abu Musa to the governorates of Basra and Kufa were made during the caliphates of Umar and Uthman , but the exact dates and circumstances are not clear. However, during the period that he was governor of one or the other of the two Muslim garrison towns in Iraq , Abu Musa is frequently mentioned in connection with the early Muslim conquest of the Sasanian Empire. In the Battle of Tostar (642) he distinguished himself as a military commander. The Persian commander, Hormuzan, had withdrawn his forces to the fortified city of Tostar. The Caliph Umar did not underestimate the strength of the enemy and he mobilized a force to confront Hormuzan. Among the Muslim forces were dedicated veterans like Ammar ibn Yasir , Al-Baraa ibn Malik al-Ansari and his brother Anas, Majra'a al-Bakri and Salamah ibn Rajaa. Umar appointed Abu Musa as commander of the army. Tostar was impossible to take by storm and several unsuccessful attempts were made to breach the walls. Fortunately, a Persian defector opened the city's gates from within making way for Abu Musa's army.
When Basra was established during 'Umar's period, he started building some canals for conveying drinking water and for irrigation. Al-Tabari reports that 'Utba ibn Ghazwan built the first canal from the Tigris River to the site of Basra when the city was in the planning stage. After the city was built, 'Umar appointed Abu Musa al-Ash'ari as the first governor. Al-Ash'ari governed during the period 17-29/638-650. He began building two important canals linking Basra with the Tigris River. These were al-Ubulla River and the Ma'qil River. The two canals were the basis for the agricultural development for the whole Basra region and used for drinking water. 'Umar also devised the policy of cultivating barren lands by assigning such lands to those who undertook to cultivate them. This policy continued during the Umayyad period and it resulted in the cultivation of large areas of barren lands through the construction of irrigation canals by the state and by individuals. 
During the time of Caliph Uthman he was replaced by Abdullah ibn Aamir as governor of Basra. interestingly he didn't show resentment about his replacement, instead he praising his Abdullah Ibn Aamir as the worthy and adequate successor. 
Following the assassination of Uthman
There are many unresolved issues regarding the First Fitna (literally "trial") period of dissension and civil war , which split the Muslim community following the assassination of the Caliph Uthman. When Ali arrived in Kufa in 656 seeking support against Aishah bint Abi Bakr and the Basrans it is agreed that Abu Musa (then the governor of Kufa), urged his subjects not to support Ali and avoid participation in the fitna . When his advice was rejected and the people of Kufa supported Ali, Abu Musa was forced to leave and Ali disposed him from his governorate.
However, the next year Abu Musa is named as the arbitrator ( hakam ) chosen by Ali's party in accordance with the terms agreed between Ali and Muawiyah after the battle of Siffin. There are many historical versions of the result of the arbitration court. According to an academic research done by Khalid Kabir Alal in University of Algeria, the most authentic version is that both Abu Musa and 'Amr ibn al-'As , the arbitrator appointed by Muawiyah I , decided that Muawiyah will be deposed, and the fate of the murderers of Uthman will be decided by the remaining of The Ten Promised Paradise .
Contributions to Islamic learning
Despite Abu Musa’s reputation as a soldier and politician, he was also praised for his beautiful recitation of the Qur'an , and he is associated with one of the early versions ( mashahef ), which was superseded by Uthman's recension. Some of the variants of Abu Musa's version have been preserved. He was also a respected faqih and was regarded among the leading judges in early Muslim history. People used to say: "The judges in this ummah are four: Umar, Ali ibn Abi Talib , Abu Musa and Zayd ibn Thabit ." Abu Musa is also credited with narrating numerous hadith, as well as being the ancestor of the founder of the Ash'ari theological school within Islam, Abu al-Hasan al-Ash'ari (d.935).
A hadith transmitted by him
Abu Musa al-Ashari reported that Muhammad said, "When a son of a servant of Allah dies, Allah Says to the angels, 'Have you taken the son of My servant?' They say, 'Yes.' Then Allah Says, 'Have you taken the fruit of his heart?' They say, 'Yes.' Allah Says, "What has My servant said?' They say, 'He has praised You and said, Inna lillahi wa inna ilaihi raji'un (Surely we belong to Allah and to Him shall we return). Then Allah Says, 'Build a house for My servant in Paradise and call it the house of praise.' From Tirmidhi, Musnad Ahmad and ibn Habban