Explanation of The Book of Divorce: Volume 2

From The Books

Concerning Ẓihār, 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.