Abd al-Rahman bin Abi Sa'id (may Allah have mercy on him) narrates from his father, Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him), that the Prophet (peace and blessings be upon him) said: "A man should not look at the private parts of another man, and a woman should not look at the private parts of another woman; and a man should not lie skin to skin with another man, and a woman should not lie skin to skin with another woman."
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 5419