Sahih Muslim Book 27, Hadith 50
We used to sell cloth in the house of Suwaid b. Muqarrin, the brother of Nu'man b. Muqarrin. There came out a slave-girl, and she said something to a person amongst us, and he slapped her. Suwaid was enraged-the rest of the hadlth is the same.
You could find no other part (to slap) but the prominent part of her face. See I was one of the seven sons of Muqarrin, and we had but only one slave-girl. The youngest of us slapped her, and Allah's Messenger (ﷺ) commanded us to set her free. 2097
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.