Sahih Muslim Book 15, Hadith Number 4078.

Chapter : How should the masters treat their slaves and expiation if they show high-handedness.

Zadhan Abl Umar reported: I came to Ibn ‘Umar as he had granted freedom to a stave. He (the narrator further) said: He took hold of a wood or something like it from the earth and said: It (freedom of a slave) has not the reward ever equal to it, but the fact that I heard Allah’s Messenger (way peace be upon him) say: He who slaps his slave or beats him, the expiation for it is that he should set him free.