Sahih Muslim, 4298

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

Rapporté par Zadhan Abi Umar : Je suis allé voir Ibn ‘Umar alors qu’il venait d’affranchir un esclave. Il a pris un morceau de bois ou quelque chose du sol et a dit : « (L’affranchissement d’un esclave) n’a même pas une récompense équivalente à cela, mais j’ai entendu le Messager d’Allah ﷺ dire : “Celui qui gifle ou frappe son esclave, l’expiation est de l’affranchir.” »