Sahih Muslim Book 15, Hadith Number 4079.
Chapter : How should the masters treat their slaves and expiation if they show high-handedness.
Zadhan reported that Ibn Umar called his slave and he found the marks (of beating) upon his back. He said to him: I have caused you pain. He said: No. But he (Ibn Umar) said: You are free. He then took hold of something from the earth and said: There is no reward for me even to the weight equal to it. I heard Allah’s Messenger (may peace be upon him) as saying: He who beats a slave without cognisable offence of his or slaps him (without any serious fault), then expiation for it is that he should set him free.