Fiqh-us-Sunnah Volume 1, Purification and Prayer, Fiqh 1.071B.

Said Anas, “When a Jewish woman was menstruating, her husband would not eat or sleep with her. The companions asked the Prophet, upon whom be peace, about that, and Allah revealed: “They question you concerning menstruation. Say: ‘It is an illness, so let women alone at such times and go not in unto them until they are cleaned. And when they have purified themselves, then go in unto them as Allah has enjoined upon you. Truly, Allah loves those who turn unto Him and loves those who have a care for cleanliness” (al-Baqarah 222). The Messenger of Allah also said, “Do everything except intercourse.” (Related by “the group,” except for al-Bukhari.)

In his comments on the subject, an-Nawawi states, “If a Muslim believes it is permissible to have intercourse with his menstruating wife, he becomes an unbelieving apostate. If he does it, not thinking that it is permissible, but out of forgetfulness or not knowing that it is forbidden or not knowing that his wife was menstruating, then there is no sin or expiation upon him. If he does it on purpose, knowing that it is forbidden, he has committed a grave sin and must repent. There are two opinions on this: the more correct one is that there is to be expiation.” He further says, “All scholars say that one may touch anything above the navel or below the knees. Most scholars say that it is permissible to touch what is between the navel and the knees, but not the vagina or anus.” An-Nawawi concludes that it is permitted but hated, as that is the strongest position from the evidence. This evidence is based upon the practice of the Prophet’s wives: when he wished to be with them during their period, they would put something over their vagina. (Related by Abu Dawud.) Al-Hafez observes, “Its chain is strong.” Masruq ibn al-Ajda’ asked ‘Aishah, “What is off limits to me sexually during my wife’s menstruation?” She said, “Nothing, except the vagina.” (Related by alBukhari in his Tarikh.)