Hadith: A man must not look at another man's ‘Awrah (must-cover body parts), nor must a woman look at another woman's ‘Awrah; neither should two men lie undressed and touch under one cover, nor should two women lie undressed and touch under one cover

عن أبي سعيد رضي الله عنه : أن رسول الله صلى الله عليه وسلم قال: «لا يَنظر الرَّجُل إلى عَوْرَة الرجل، ولا المرأة ُإلى عَوْرَة المرأة،ِ ولا يُفْضِي الرَّجُلُ إلى الرَّجُل في ثوب واحد، ولا تُفْضِي المرأةُ إلى المرأةِ في الثوب الواحد».
[صحيح] - [رواه مسلم]

Abu Sa‘īd (may Allah be pleased with him) reported that the Messenger of Allah (may Allah's peace and blessings be upon him) said: "A man must not look at another man's ‘Awrah, nor must a woman look at another woman's ‘Awrah; neither should two men lie undressed and touch under one cover, nor should two women lie undressed and touch under one cover.”
Sahih/Authentic. - [Muslim]

Explanation

A man must not look at another man's ‘Awrah and a woman must not look at another woman's ‘Awrah. This is a prohibition for the one looking not to look at the ‘Awrah of another. Say, for example, that a woman revealed her ‘Awrah for a necessity, such as medical treatment at a physician's office, and her sister was with her, then it is not permissible for her sister to look at her ‘Awrah. Or if her ‘Awrah was exposed by the blowing wind, etc. the other woman must not look at what is between the navel and the knees. The same ruling applies to men; a man must not look at the ‘Awrah of another man, which is the area between the navel and the knees. If a man's ‘Awrah was exposed for a necessity or unintentionally, then it is not permissible for others to look at it. If a man's eyes fell on the ‘Awrah of his brother all of a sudden, then he must avert his eyes and not continue to look. "Neither should two men lie undressed and touch under one cover, nor should two women lie undressed and touch under one cover," i.e. the bare skins of the two naked men or women must not touch, for when two naked people share the same cover, their ‘Awrahs might come in contact. Such contact is prohibited just as looking at them. Rather, touching the ‘Awrah is more emphatically prohibited than looking at it. Men and women are the same in this regard based on the text of the Hadīth.