Sahih Muslim Book - 3, Hadith - 768

حَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ حَدَّثَنَا زَيْدُ بْنُ الْحُبَابِ عَنْ الضَّحَّاكِ بْنِ عُثْمَانَ قَالَ أَخْبَرَنِي زَيْدُ بْنُ أَسْلَمَ عَنْ عَبْدِ الرَّحْمَنِ بْنِ أَبِي سَعِيدٍ الْخُدْرِيِّ عَنْ أَبِيهِ أَنَّ رَسُولَ اللَّهِ صَلَّى اللَّهُ 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.

It was narrated from 'Abdur-Rahman bin Abi Sa'eed Al-Khudri, from his father, that the Messenger of Allah (s.a.w) said: "No man should look at the 'Awrah of another man, and no woman should look at the 'Awrah of another woman. No man should lie with another man under the same cover, and no woman should lie with another woman under the same cover."