What is the expiation for someone who swore by saying: "It is unlawful for me to do such and such" while he is married?

If someone swears to forbid what is lawful, and it is a specific thing other than his wife, then it is considered a vain oath (laghw) and nothing is binding on him according to Malik and Shafi'i. Others said he has the choice between abandoning it or offering an expiation for an oath (kaffarat yamin).

However, if he swears by forbidding generally, such as saying: "What Allah has made lawful for me is forbidden," and he has a wife, the Malikis hold that divorce becomes binding on him unless he explicitly excludes his wife by word or intention. The Hanbalis believe it is a form of Dhihar (injurious assimilation), and he must offer kaffarat Dhihar if he intended to forbid his wife. Some of them said it is a metaphorical expression (kinayah), so it would be a divorce by intention. As for the Hanafis, his wife does not become involved unless by intention; if he intended divorce, it is a divorce, and if people commonly understand this phrase as divorce, then it is a divorce. Sheikh al-Islam Ibn Taymiyyah holds that what is binding on him is an expiation for an oath, and if he offers the expiation for Dhihar, it is better. This latter opinion is the closest to correctness.