Sahih Muslim | Book 3 | Chapter 16

Narrated by 'Abd al-Rahman, the son of Abu Sa'id al-Khudri,
'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.

Narrated by
This hadith has been narrated by Ibn Abu Fudaik and Dabbik b. 'Uthman with the same chain of transmitters and they observed: Private parts of man are the nakedness (which is concealed).