It is narrated from Sayyidah Umm Salamah (may Allah be pleased with her) that she heard the Messenger of Allah (peace and blessings be upon him) say: "When the mukatab (a slave who has contracted to buy his freedom) has enough wealth with which he can pay the price of his freedom, then observe hijab from him."
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 19980