Abu Dawud Book 8, Hadith Number 2694.
Chapter : Not known.
Narated By Ali ibn AbuTalib : Some slaves (of the unbelievers) went out to the Apostle of Allah (PBUH) on the day of al-Hudaybiyyah before treaty. Their masters wrote to him saying: O Muhammad, they have not gone out to you with an interest in your religion, but they have gone out to escape from slavery. Some people said: They have spoken the truth, Apostle of Allah, send them back to them. The Apostle of Allah (PBUH) became angry and said: I do not see your restraining yourself from this action), group of Quraysh, but that Allah send someone to you who strike your necks. He then refused to return them, and said: They are emancipated (slaves) of Allah, the Exalted.