Chapter 33: Hot Baths (Kitab Al-Hammam)
Narrated Abu Sa' id al-Khudri: The Prophet (ﷺ) said: A man should not look at the private parts of another man, and a woman should not look at the private parts of another woman. A man should not lie with another man without wearing lower garment under one cover; and a woman should not be lie with another woman without wearing lower garment under one cover.
Sunan Abi Dawud 4018