Sahih Muslim Book 15, Hadith Number 4082.

Chapter : How should the masters treat their slaves and expiation if they show high-handedness.

Hilal b. Yasaf reported that a person got angry and slapped his slave-girl. Thereupon Suwaid b. Muqarrin said to him: 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 (may peace be upon him) commanded us to set her free.2097