Jami` at-Tirmidhi 2793

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