The Book of Oaths (Kitab Al-Aiman)
Muslim :: Book 15 : Hadith 4081
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.