Abu Dawud Book 36, Hadith Number 5148.

Narated By Mu’awiyah ibn Suwayd ibn Muqarrin : I slapped a freed slave of ours. My father called him and me and said: Take retaliation on him. We, the people of Banu Muqarrin, were seven during the time of the Prophet (PBUH),and we had only a female servant. A man of us slapped her. The Apostle of Allah (PBUH) said: Set her free. They said: We have no other servant than her. He said: She must serve them till they become well off. When they become well off, they should set her free.