Is sexual intercourse with one's wife, after the husband has said to her: "You are forbidden to me like my mother's breast," considered lawful, and what are the consequences of this act?

There is no blame on the husband if he had intercourse with his wife while ignorant of the ruling. However, the majority of jurists believe that ẓihār (a form of divorce) occurs with the statement "You are to me like my mother from behind," and expiation is due upon returning (to the wife). Scholars have differed on this matter: the Hanafis and some Shafi'is believe that ẓihār does not occur with such a phrase if the comparison is made with a body part that is permissible to look at. In contrast, the Malikis believe that ẓihār occurs by comparing with any part of the mother. The Shafi'is differentiate between parts that are mentioned for honor or not. As for the Hanbalis, they believe that ẓihār occurs by comparing with the permanent parts of the mother. The preponderant opinion is that ẓihār occurs with this phrase, and the expiation for ẓihār, mentioned in the Quran, is due: freeing a slave, or fasting for two consecutive months, or feeding sixty needy persons.