Abu Dawud Book 24, Hadith Number 3917.

Chapter : A slave who has entered into an agreement to purchase his freedom mukatab and made a part payment but finally he is unable to complete the amount or dies.

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.