Abu Dawud Book 26, Hadith Number 4007.

Narated By Abu Sa’id al-Khudri : The Prophet (PBUH) 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. A man should not lie with another man without wearing lower garment under one cover; and a woman should not be lie with another woman without wearing lower garment under one cover.