Musnad Abu Ya'la al-Mawsili 109:61

Amr al-Naqid narrated to us, Ali ibn Thabit al-Jazari narrated to us, al-Wazi' ibn Nafi' narrated to us, from Abu Salamah ibn Abd al-Rahman, from Jabir, who said: We were praying with the Messenger of Allah (may Allah bless him and grant him peace) during the Battle of Badr, when he smiled during his prayer. When he finished the prayer, we said: "O Messenger of Allah, we saw you smile." He said: "Michael passed by me, and upon his wing was the trace of dust, as he was returning from pursuing the people. So he smiled at me, and I smiled at him."