Narrated 'Abdur-Rahman bin Abi Sa'eed [Al-Khudri]from his father who said:
"The Messenger of Allah (ﷺ) said: 'A man is not to look at the 'Awrah of a man, and a woman is not to look at the 'Awrah of a woman. A man is not to be alone with a man under one garment, and a woman is not to be alone with a woman under one garment.'" (JA)
English reference (JA): Vol. 5, Book 41, Hadith 2793 (JA)
Arabic reference (JA): Book 43, Hadith 3023 (JA)