Abu Dawud Book 27, Hadith Number 4093.
Chapter : A slave is allowed to see the unveiled head i.e. hear of his mistress.
Narated By Jabir : Umm Salamah asked the Apostle of Allah (PBUH) permission for getting herself cupped. He commanded Abu Tibah to cup her. The translator said: I think he was her foster brother or a boy not yet of age.