Chapter# 16, Book 3, Number 0667:
‘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.
Book 3, Number 0668:
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).