Sunan Abi Dawud 4018
سنن أبي داود#4018SahihNarrated AbuSa'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
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Al-AlbaniSahih
Muhammad Muhyi Al-Din Abdul HamidSahih
Zubair Ali ZaiSahih Muslim (338)
In-book reference: 33:10