What is the ruling on a man saying to his wife: "You are forbidden to me like my mother's back," and what is its expiation?

If a man makes ẓihār conditional on something, and that thing occurs, he is considered to have committed ẓihār against his wife. If he combines ẓihār with the word "prohibition" (تحريم), the most authoritative opinion is that he is only bound by the ẓihār. He is not permitted to enjoy his wife until he expiates. The expiation for ẓihār is 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, based on the Almighty's saying: "And those who commit ẓihār from their wives and then intend to revoke what they said - then [there must be] the freeing of a slave before they touch one another. That is [said] to remind you. And Allah is Acquainted with what you do. And he who does not find [a slave] - then a fast for two consecutive months before they touch one another. And he who is unable to do so - then the feeding of sixty poor persons."