(134) باب فِي حَقِّ الْمَمْلُوكِ
Zadhan said:
I came to Ibn 'Umar when he set his slave free. He took a stick or something else from the earth and said; for me there is no reward even equivalent to this. I heard the Messenger of Allah (May peace be upon him) say: If anyone slaps or beats his slave the atonement due from him is to set him free.
Reference: Sunan Abi Dawud 5168
In-book reference: Book 43, Hadith 396
English translation: Book 42, Hadith 5149