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.'" (IT)
English reference (IT): Vol. 5, Book 41, Hadith 2793 (IT)
Arabic reference (IT): Book 43, Hadith 3023 (IT)