(8) باب صُحْبَةِ الْمَمَالِيكِ وَكَفَّارَةِ مَنْ لَطَمَ عَبْدَهُ
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 (ﷺ) as saying: He who beats a slave without cognizable offence of his or slaps him (without any serious fault), then expiation for it is that he should set him free.
Reference: Sahih Muslim 1657 b
In-book reference: Book 27, Hadith 47
USC-MSA web (English) reference(deprecated numbering scheme): Book 15, Hadith 4079