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."[Muslim]. (ID)
App reference (ID): Book 18, Hadith 117 (ID)
Arabic/English book reference (ID): Book 18, Hadith 1627 (ID)