It is narrated from Hazrat Umm Salamah (may Allah be pleased with her) that the Prophet (peace and blessings be upon him) said: When any slave of yours among the women becomes a mukatab (contracted for freedom) and he possesses enough to pay the amount of his contract and thus gain his freedom by handing it over to his owner, then that woman should observe veil from her slave.
Hadith Referenceمسند احمد / مسند النساء / 26629
Hadith Gradingحکم دارالسلام:إسناده ضعيف لجهالة نبهان، وقد تفرد بهذا الحديث، ومما يدل على ضعف هذا الحديث عمل السيدة عائشة بخلافه