Sahih Adab al-Mufrad

It is narrated from Abu ‘Umar Zadhan, he states that we were with Sayyiduna Ibn ‘Umar (may Allah be pleased with them). He called his slave whom he had beaten. He removed the cloth from his back and said: “Are you in pain?” He replied: “No.” So he freed the slave, then picked up a straw from the ground and said: “I will not receive a reward even equal to the weight of this straw.” I said: “O Abu ‘Abdur-Rahman! Why do you say this?” He said: “I heard the Prophet (peace and blessings of Allah be upon him), he used to say or he said: ‘Whoever punishes his slave without guilt or slaps him on the face’—and in another narration: ‘Whoever slaps his slave on the face or imposes a punishment on him for something he did not commit, its expiation is that he should set him free.’”