Abu Dawud Book 24, Hadith Number 3921.

Chapter : Emancipation of a slave on a certain condition.

Narated By Umm Salamah, Ummul Mu’minin : Safinah said: I was a slave of Umm Salamah, and she said: I shall emancipate you, but I stipulate that you must serve the Apostle of Allah (PBUH) as long as you live. I said: Even if you do not make a stipulation, I shall not leave the Apostle of Allah (PBUH). She then emancipated me and made the stipulation with me.