It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: "When a man buys a slave-girl, there is no harm in his examining her carefully; he may look at whatever he wishes except her awrah, and the awrah of a slave-girl is between her knees and her navel."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3224