Musnad Ahmad 1097:1

Yazid narrated to us, Muhammad ibn Ishaq informed us, from his father, who said: Abu Dawud al-Mazini said, and Yazid narrated to us, he said: Muhammad said, so my father narrated to me, from a man from Banu Mazin, from Abu Dawud al-Mazini, and he had witnessed Badr, he said: "I was pursuing a man from the polytheists to strike him, when his head fell off before my sword reached him, so I knew that someone other than me had killed him."