Sahih Muslim Book 3, Hadith Number 667.

Chapter : It Is forbidden to see the private parts of someone else.

‘Abd al-Rahman, the son of Abu Sa’id al-Khudri, reported from his father: The Messenger of Allah (may peace be upon him) said: A man should not see the private parts of another man, and a woman should not see the private parts of another woman, and a man should not lie with another man under one covering, and a woman should not lie with another woman under one covering.