Musnad Ahmad 788:83

Ali ibn Abdullah narrated to us, Al-Walid ibn Muslim narrated to us, Yahya ibn Abd al-Aziz al-Urduni narrated to us, from Abdullah ibn Nu'aym al-Qayni, who said: Al-Dahhak ibn Abd al-Rahman ibn Arzab al-Ash'ari narrated to me, that Abu Musa narrated to them, saying: When Allah, the Almighty and Majestic, defeated Hawazin at Hunayn, the Messenger of Allah (peace and blessings of Allah be upon him) appointed Abu Amir al-Ash'ari as commander over the pursuit cavalry. He pursued them, and I was among those who pursued them. His horse sped up with him, and he caught up with Ibn Durayd ibn al-Simmah. He killed Abu Amir and took the banner. I charged at Ibn Durayd and killed him, then I took the banner and returned with the people. When the Messenger of Allah (peace and blessings of Allah be upon him) saw me carrying the banner, he said: "O Abu Musa, has Abu Amir been killed?" I said: "Yes, O Messenger of Allah." He said: I saw the Messenger of Allah (peace and blessings of Allah be upon him) raise his hands, saying: "O Allah, Your servant, Your servant Abu Amir, make him among the most numerous on the Day of Resurrection."