What is the ruling on a wife going to her father's house after her husband swore to her saying: "It is unlawful for me from you, that you do not go downstairs starting from this moment for 30 days," and he intended to prevent her from going to her father's house?

Scholars differ on the ruling concerning swearing by what is unlawful (al-haram). Some hold it to be ẓihār (a form of divorce), some hold it to be ṭalāq (divorce), and some hold it to be an oath (yamīn). The relied-upon opinion differentiates based on whether one intended ṭalāq, ẓihār, or an oath by saying "al-haram."

If you intended ṭalāq, then if your wife goes to her family's house before thirty days, a divorce occurs. You may take her back (perform rajʿah) if this divorce does not complete the three divorces.

If you intended ẓihār, then ẓihār occurs, and it becomes unlawful for you to have intercourse with her until you expiate for ẓihār, as mentioned in Surat Al-Mujadilah, which is: freeing a slave; if one cannot find one, then fasting for two consecutive months; if one is unable to do so, then feeding sixty poor people.

If you intended it as an oath or did not intend anything specific, then it is considered an oath. In this case, if your wife goes to her family's house, no divorce or ẓihār occurs. You are required to pay the expiation for an oath (kaffārat yamīn): feeding ten poor people or clothing them. If you cannot find that, then fasting for three days.

Swearing by what is unlawful is not permissible. The permissible oath is to swear by Allah the Almighty.