Musnad Abu Ya'la al-Mawsili 243:2

Dawud ibn 'Amr ibn Zuhayr al-Dabbi narrated to us, al-Walid ibn Muslim narrated to us, from Yahya, from 'Abd Allah ibn Nu'aym, from al-Dahhak ibn 'Abd al-Rahman ibn 'Arzab al-Ash'ari, from Abu Musa al-Ash'ari: That the Messenger of Allah (may Allah bless him and grant him peace) appointed Abu 'Amir al-Ash'ari on the day of Hunayn over the cavalry of pursuit. When Hawazin was defeated, he pursued them until he caught up with Durayd ibn al-Simmah, and he hastened toward him himself. The son of Durayd killed Abu 'Amir. Abu Musa said: So I charged at the son of Durayd and killed him, and I took the banner and returned with the people to the Messenger of Allah (may Allah bless him and grant him peace). When he saw the banner in my hand, he said: "O Abu Musa, has Abu 'Amir been killed?" I said: "Yes, O Messenger of Allah." He said: So he raised his hands, supplicating for him, saying: "O Allah, Abu 'Amir, place him among the most numerous on the Day of Resurrection," or something similar to this.