Sahih Adab al-Mufrad

Hilal bin Yasaf says that we were selling cloth in the house of Suwaid bin Muqarrin. A slave girl came out and said something to a man. That man slapped the slave girl, so Suwaid bin Muqarrin said to him: Did you slap her on the face? I found myself to be one of seven persons (i.e., we were seven men) and we had only one servant. One of us slapped her, so the Prophet (ﷺ) ordered that she be set free.

I slapped a poor servant of ours, so my father called me and said (to him): Retaliate. We, the sons of Muqrin, were seven, and we had a servant. One of us slapped her, and that was mentioned to the Prophet ﷺ, so he said: "Order them to set her free." It was said to the Prophet ﷺ: They have no other servant besides her. He said: Then let them make use of her, and when they no longer need her, let her go free. / 178.

132c/176 And in another narration from Abu Shu'bah from Suwayd ibn Muqarrin al-Muzani—he saw a man slap his servant—he said: "Do you not know that striking the face is forbidden? I saw myself, and I was the seventh of seven brothers, in the time of the Messenger of Allah ﷺ, and we had nothing but one servant. One of us slapped him, so the Prophet ﷺ ordered us to set him free." / 179.

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.’”