From Sahih Muslim (Hadith 4304):
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
Reference: Muslim 4304