The expedition (sariyyah) of Abu Musa al-Ashari, took place as consequence of the league of tribes who had mobilized to attack the Muslim community. With the Muslims having the upper-hand at the battle of Hunayn, the enemy retreated to Awtas area to mobilize, attack and use this place for military purposes against the Muslims.
While, Abu Amir, was commanded to pursue some of the enemy, who fled the battle of Hunain, he was killed by the enemy. So, Abu Musa al-Ashari, the nephew of Abu Amir took his command and chased the perpetrator and killed him.
“The Prophet sent me with Abu ‘Amir. Abu Amir was shot at his knee with an arrow which a man from Jushm had shot and fixed into his knee. I went to him and said, “O Uncle! Who shot you?” He pointed me out (his killer) saying, “That is my killer who shot me (with an arrow).” So I headed towards him and overtook him, and when he saw me, he fled, and I followed him and started saying to him, “Won’t you be ashamed? Won’t you stop?” So that person stopped, and we exchanged two hits with the swords and I killed him.
Then I said to Abu ‘Amir. “God has killed your killer.” He said, “Take out this arrow” So I removed it, and water oozed out of the wound. He then said, “O son of my brother! Convey my compliments to the Prophet and request him to ask God’s Forgiveness for me.” Abu Amir made me his successor in commanding the people (i.e. troops). He survived for a short while and then died. …” (Sahih al-Bukhari volume 5, Book 59, Hadith 612)
“He (the Prophet) sent me along with Abu ‘Amir and Abu ‘Amir received a wound in his knee from the arrow, (shot by) a person of Bani Jusham. It stuck in his knee. I went to him and said: Uncle, who shot an arrow upon you? Abu ‘Amir pointed out to Abu Musa and said:
Verily that one who shot an arrow upon me in fact killed me. Abu Musa said: I followed him with the determination to kill him and overtook him and when he saw me he turned upon his heels. … He stopped and I had an encounter with him and we exchanged the strokes of (swords). I struck him with the sword and killed him.
And he said: Now draw out this arrow. I drew out the arrow and there came out from that (wound) water. Abu ‘Amir said: My nephew, go to God’s Messenger and convey my greetings to him and tell him that Abu Amir begs you to ask forgiveness for him. And Abu Amir appointed me as the chief of the people and he died after a short time.” (Sahih Muslim Book 31, Hadith 6092)
“The Messenger of God sent me with Abu ‘ Amir. When Abu ‘ Amir was hit by an arrow, which stuck in his knee,shot by a man from the Banu Jusham, I came to him and said, “O uncle, who shot you?” Pointing to the man, he said, “That one is my killer! You see him, that one shot me!” Abu Musa went after him and overtook him, but when the man saw him he fled. Abu Musa pursued him, … So he turned back.
They confronted each other and exchanged two blows. Abu Musa killed him with his sword and returned to Abu ‘ Amir telling him that God had killed his enemy. Abu ‘Amir asked him to remove the arrow [from his knee], and when he removed it water flowed from [the wound]… He then appointed Abu Musa in charge of the Army and died shortly thereafter…” 
“The retreating enemy troops fled from Hunayn for Ta’if, Nakhla or Awtas. A group of Muslims headed by Abu Amir al-Ash’ari, were despatched to chase the enemy. In Skirmishes which followed Abu Amir was martyred. Similarly another group of horsemen pursued the idolaters heading to Nakhlah.” 
 Al-Tabari: The Last Years of the Prophet, volume 9, page 17
 Notes On Entering Deen Completely: Success, Ummah, Renewal By Talib Jaleel, page 511