it is forbidden for a muslim man to have sexual intercourse with his wife when she is menstruating. this is clear in the following verse of the qur'an:
"they ask thee concerning menstruation. say: it is a harm. so keep away from women in
their courses, and do not approach them until they are
clean. but when they have purified themselves, ye may
approach them in any manner, time, or place ordained for
you by allah. for allah loves those who turn to him
constantly and he loves those who keep themselves pure
and clean." [al-baqarah, 2:222]
there are also hadith about this, among them:
first: "whoever has sexual intercourse with a menstruating woman, or a woman in her anus, or approaches a soothsayer and believes what he is told has disbelieved in that which was revealed to muhammad."
second: on the authority of anas ibn malik, who said: "when one of their women has their period, the jews used to put her out of the house, and they would not eat, drink, or sleep with her in the house. the prophet was asked about this, and allaah revealed the verse:
"they ask thee concerning women's courses. say: they are a
hurt and a pollution: so keep away from women in their courses, ...
then the prophet said: "be with them in the house, and do everything except for intercourse itself." the jews said: "this man wants to leave nothing which we do without doing something different." then, asyad ibn hudair said: "o messenger of allah, verily the jews says such-and-such, should we not then have sexual intercourse during menstruation?" the prophet's face changed such that they thought that he was enraged with them, so they left. as they were coming out, they saw a gift of milk being brought to the prophet. the prophet then sent someone after them to give them a drink of milk, so they felt that he was not actually angry with them." [muslim, abu 'auwaana and abu daawood].