Muslim 768

From Sahih Muslim (Hadith 768):

Abd al-Rahman, the son of Abu Sa'id al-Khudri, reported from his father:The Messenger of Allah (ﷺ) 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