Amr bin Shu'ayb narrates from his father, and he from his grandfather (may Allah be pleased with him), that the Prophet (peace and blessings be upon him) said: "When one of you marries his slave-girl to his slave or to his hired worker, then let him never look at her awrah."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3219