What is the ruling on someone who jokingly said to his wife, "You are forbidden to me today," and then had intercourse with her?

Divorce, as well as ẓihār (a pre-Islamic form of divorce) and oaths, takes effect whether uttered in jest or in earnest. Thus, if a man says to his wife, "You are forbidden to me," and does not intend anything specific, he must offer expiation for a broken oath. However, if he intended divorce or ẓihār by his words, then what he intended takes effect, even if he was joking. Anything that Allah has made lawful, if a person declares it forbidden upon himself, then its ruling is that of an oath: he must offer expiation for a broken oath, and then he may do it.