Musnad Ahmad 1153:377

Abu al-Mundhir narrated to us, Malik narrated to us, from al-Fudayl ibn Abi Abd Allah, from Abd Allah ibn Nayyar al-Aslami, from Urwah, from Aisha, that a man followed the Messenger of Allah, peace and blessings of Allah be upon him, and said: "I will follow you so that I may attain [the reward/spoils] with you." The Messenger of Allah, peace and blessings of Allah be upon him, said: "Do you believe in Allah and His Messenger?" He said: "No." He said: "Then we do not seek the help of a polytheist." He said: Then he said to him a second time: "Do you believe in Allah and His Messenger?" He said: "Yes." So he set off, and he followed him.