The Messenger of Allah ﷺ said, "A man must not look at the private parts of another man, nor a woman at the private parts of another woman. A man must not lie with another man under a single garment, nor a woman with another woman under a single garment."
أن رسول الله ﷺ قال: لا ينظر الرجل إلى عورة الرجل، ولا المرأة إلى عورة المرأة، ولا يفضي الرجل إلى الرجل في ثوب واحد، ولا تفضي المرأة إلى المرأة في الثوب الواحد.
Muslim narrated it in the Book of Menstruation (338), from Abu Bakr bin Abu Shayba, who said, Zayd bin Hubab related to us, from Dahhak bin Uthman, who said, Zayd bin Aslam informed me, from Abdul-Rahman bin Abu Saeed, from his father, and he mentioned it.There is no disagreement among the scholars regarding the prohibition of a man looking at the private parts of another man, and the prohibition of a woman looking at the private parts of another woman, and likewise the prohibition of a man looking at the private parts of a woman, and a woman looking at the private parts of a man. Excepted from this are spouses, for each of them is permitted to look at the private parts of the other.As for a man looking at his female relatives who are unmarriageable to him, and their looking at him, one view holds that nothing is permissible except what appears during service and ordinary activity, as Nawawi mentioned in his commentary on Sahih Muslim. He also said, "The sahih view is that it is permissible above the navel and below the knee." End of quote.I say, the more a woman remains modestly covered, even in front of her unmarriageable male relatives, the better and more protective it is. For Satan flows through a person like blood runs through the veins, and there has been much occurrence of wrongdoing between such relatives in this age of moral decay, as we hear. We ask Allah for well-being and safety.As for his statement, "it is permissible above the navel and below the knee," this expansive view was derived from a daif hadith, and it is the opinion held by most jurists. I have discussed it in the Book of Clothing, so refer to it there.