Sahih Muslim Book 15, Hadith Number 4081.

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

Mu’awiya b. Suwaid reported: I slapped a slave belonging to us and then fled away. I came back just before noon and offered prayer behind my father. He called him (the slave) and me and said: Do as he has done to you. He granted pardon. He (my father) then said: We belonged to the family of Muqarrin during the lifetime of Allah’s Messenger (may peace be upon him and had only one slave-girl and one of us slapped her. This news reached Allah’s Apostle (may peace be upon him) and he said: Set her free. They (the members of the family) said: There is no other servant except she. Thereupon he said: Then employ her and when you can afford to dispense with her services, then set her free.