Musannaf Ibn Abi Shayba 7:469

Waki’ narrated to us, from Sufyan, from Firas, from Abu Salih, from Zadhan, from Ibn Umar, that he emancipated a slave of his, then he took something from the earth and said: "I do not have from his reward [anything] like this." I heard the Prophet, peace and blessings of Allah be upon him, saying: "Whoever slaps a servant of his, his expiation is to emancipate him."