Chapter 28: The Book of Oaths
Suwaid b. Muqarrin reported that he had a slave-girl and a person (one of the members of the family) slapped her, whereupon Suwaid said to him: Don' t you know that it is forbidden (to strike the) face. He said: You see I was the seventh one amongst my brothers during the lifetime of Allah' s Messenger (ﷺ), and we had but only one servant. One of us got enraged and slapped him. Thereupon Allah' s Messenger (ﷺ) commanded us to set him free.
Sahih Muslim 1658d