It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that the Prophet (peace and blessings be upon him) said: "There is no harm in a man turning a slave girl about to examine her when he wants to buy her, except for her awrah, which is from her knees to where the waist-wrap is tied."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 10727