Abu Dawud
Book Of Emancipation Of Slaves
Book Number: 024 / Hadith Number: 3917
Narated By Umm Salamah, Ummul Mu'minin : The Apostle of Allah (pbuh) said to us: If one of you has a slave, and he enters into an agreement to purchase his freedom, and can pay the full price, she must veil herself from him.