What is the ruling on swearing by "bil-haram" (by what is forbidden), which is common among the general populace in Tunisia, such as one saying: "Bil-haram, I will not do such and such"?

Scholars have differed regarding the ruling on a man who tells his wife, "You are unlawful to me," due to the absence of an explicit text. The opinion closest to the evidence is that making one's wife unlawful incurs the expiation of ẓihār. This is the view of Imam Ahmad and Isḥaq, and it was the fatwa (definition) given by Uthman (may Allah be pleased with him), because the meaning of "You are to me as the back of my mother" is "You are unlawful to me," and the Quran has explicitly mentioned the expiation for ẓihār.

Following this opinion in strength is the view of performing the expiation for an oath (kaffārat al-yamīn) and seeking forgiveness.

Based on this, if someone swears by making something unlawful that he will or will not do something, then breaks his oath, and he has a wife, he is subject to what is required of one who performs ẓihār: freeing a slave. If he cannot find one, then fasting for two consecutive months. If he is unable to do that, then feeding sixty poor people. Intercourse and its preliminaries are forbidden to him before expiation. If he also performs the expiation for an oath, it would be more cautious.