Sahih Muslim
The Book of Oaths (Kitab Al-Aiman)
Book Number: 015 / Hadith Number: 4078
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 evert 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.