Hadith: A man must not look at another man's ‘Awrah (must-cover body part), nor must a woman look at another woman's ‘Awrah

عَن أَبِي سَعِيدٍ الْخُدْرِيِّ رضي الله عنه أَنَّ رَسُولَ اللهِ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ قَالَ:
«لَا يَنْظُرُ الرَّجُلُ إِلَى عَوْرَةِ الرَّجُلِ، وَلَا الْمَرْأَةُ إِلَى عَوْرَةِ الْمَرْأَةِ، وَلَا يُفْضِي الرَّجُلُ إِلَى الرَّجُلِ فِي ثَوْبٍ وَاحِدٍ، وَلَا تُفْضِي الْمَرْأَةُ إِلَى الْمَرْأَةِ فِي الثَّوْبِ الْوَاحِدِ».
[صحيح] - [رواه مسلم] - [صحيح مسلم: 338]

Abu Sa‘īd al-Khudri (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 (must-cover body part), nor must a woman look at another woman's ‘Awrah; neither should two men lie undressed under one attire, nor should two women lie undressed under one attire."
Sahih/Authentic. - [Muslim]

Explanation

The Prophet (may Allah's peace and blessings be upon him) forbade the man from looking at another man's ‘Awrah and the woman from looking at another woman's ‘Awrah.
‘Awrah: it is whatever one feels shy if uncovered. Man's ‘Awrah is the area between his navel and the knee. As for the woman, she is all ‘Awrah with regards to foreign men, but with regards to other women and her Mahram men, she can uncover what is usually exposed while doing house chores.
Moreover, the Prophet (may Allah's peace and blessings be upon him) forbade the man from lying undressed with another man in one garment or under one cover, and the woman from lying undressed with another woman in one garment or under one cover because this can lead each of them to touch the ‘Awrah of the other, which is forbidden, just as looking at the ‘Awrah is forbidden; rather, touching the ‘Awrah is more strictly forbidden given the fact that it leads to graver consequences.