It is narrated from Sayyidah Umm Salamah (may Allah be pleased with her) that the Prophet (peace and blessings be upon him) said: "When one of you (women) has a mukatab slave, and he has with him (the remainder) that he is to pay, let her observe hijab from him."
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 20257