Sahih Muslim 768
صحيح مسلم#768SahihAbd 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
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Every narration in Sahih Muslim is accepted as Sahih (authentic) by the established status of the book.
In-book reference: 3:90