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 (ﷺ) 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 : Book 43, Hadith 396 English translation : Book 42, Hadith 5149
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