Musnad Ahmad 30:727

Hammad ibn Usamah narrated to us, Ismail ibn Abi Khalid informed us, from Qays, from Abu Hurayrah, who said: When I arrived to the Prophet, may Allah bless him and grant him peace, I said on the way: "Oh, what a night, in its length and its hardship, Yet it has escaped from the abode of disbelief." He said: And a servant of mine ran away from me on the way. He said: When I arrived to the Messenger of Allah, may Allah bless him and grant him peace, I pledged allegiance to him. While I was with him, the servant appeared. The Messenger of Allah, may Allah bless him and grant him peace, said to me: "O Abu Hurayrah, this is your servant." I said: "He is for the sake of Allah," and I freed him.