Explanation of The Book of Divorce By Shaykh Muhammad bin Salih al-Uthaymin Book 2

Concerning zihar, it refers to a man's likening his wife to a woman [who is permanently and absolutely unlawful for him] intending by this to make his wife unlawful for him. This is like saying to her: "You are to me like my mother's back", "You are to me like my sister's back", or "You are to me like your mother's back", for a man's mother-inlaw is permanently unlawful for him. Still, if he says to his wife: "You are to me like your sister's back", this is not Ẓihār according to the right view [since a man's sister-in-law is temporarily, not permanently, unlawful for him]. Some scholars maintain that even if he likens her to a woman who is temporarily unlawful for him, this is Ẓihār, but this is not a correct view. The right view is that Ẓihār.