What is the ruling of Islamic law regarding a man's statement to his wife: "You are forbidden to me if I return to smoking," and then he returns to smoking, and what is the expiation for that?

The pronouncement that was issued, which prohibits your wife to you upon your return to smoking, means that if you revert to it, the predominant ruling is that the prohibition of the wife depends on the husband's intention. If you intended divorce, it takes effect. If you intended zhihar (likening one's wife to an unmarriageable relative), then its expiation is binding upon you. If you intended an oath or did not intend anything, then the expiation of an oath is binding upon you. One must be cautious of anger and of resolving problems through divorce or prohibition. Smoking is forbidden, and one must be wary of it and repent from consuming it.