Abu Dawud Book 27, Hadith Number 4094.

Chapter : A slave is allowed to see the unveiled head i.e. hear of his mistress.

Narated By Anas ibn Malik : The Prophet (PBUH) brought Fatimah a slave which he donated to her. Fatimah wore a garment which, when she covered her head, did not reach her feet, and when she covered her feet by it, that garment did not reach her head. When the Prophet (PBUH) saw her struggle, he said: There is no harm to you: Here is only your father and slave.