Riyad as-Salihin: The Book of the Prohibited actions – Hadith No. 948 — Abu Sa’id Al-Khudri (May Allah be pleased with him) said: The Messenger of… (Hadith No. 948)

Abu Sa'id Al-Khudri (May Allah be pleased with him) said: The Messenger of Allah (ﷺ) said, "A man must not look at a man's private parts nor must a woman look at a woman's private parts; neither should two men lie naked under one cover, nor should two women lie naked under the same cover.".

Explanation:

Reference: Riyad as-Salihin, The Book of the Prohibited actions, Hadith No. 948