Musnad Ahmad 1153:1140

'Abd al-Rahman narrated to us, Malik narrated to us, from Fudayl ibn Abi 'Abd Allah, from 'Abd Allah ibn Nayyar, from 'Urwah, from 'A'ishah: That the Messenger of Allah (peace and blessings of Allah be upon him) went out to Badr. A man from the polytheists followed him, and he caught up with him at the Jamrah. He said, "I wanted to follow you and obtain [spoils] with you." He said, "Do you believe in Allah, the Almighty and Majestic, and His Messenger?" He said, "No." He said, "Return, for we will not seek the help of a polytheist." He said: Then he caught up with him at the tree—and the companions of the Messenger of Allah (peace and blessings of Allah be upon him) were happy about that, for he had strength and endurance—and he said, "I have come to follow you and obtain [spoils] with you." He said, "Do you believe in Allah and His Messenger?" He said, "No." He said, "Return, for I will not seek the help of a polytheist." He said: Then he caught up with him when he appeared upon the desert, and he said to him the same thing. He said, "Do you believe in Allah and His Messenger?" He said, "Yes." He said, "Then come out [with us]."