Ан-Нур 24:30

By way of guidance as to how men and women should behave at home and in society, two basic injunctions have been given here—one defines that portion of the body which must be covered (satr), while the other advises that the eyes should be cast down. That portion of a male’s body, which is required to be kept covered at all times, is from the navel down to the knees, including the knees. This is satr, which is obligatory before all except one’s wife, or where some need arises by which prohibitions are relaxed, for instance, in the case of medical examinations. The second important thing is that when a man and a woman happen to confront each other, the man should keep his eyes cast down. A casual meeting between a man and a woman who are not married or who are not very close relatives, should never be as informal as that between any two men. During any casual meeting between a man and a woman, the man should keep his eyes down and if a man happens to accidentally cast a glance at an unknown woman, he should immediately direct his gaze elsewhere and should not deliberately look at her again. The instructions about looking down and concealing the private parts of the body are also applicable to women, as is made clear in the next verse (Verse 31).