What is the ruling on someone who swore an oath of divorce saying: "If I return to smoking, my wife shall be unlawful to me as my mother's back," then he returned to smoking? Is he liable for an expiation of Zihar (analogous divorce) or an expiation for breaking an oath? And what is the expiation?

This statement is a suspended ẓihār contingent on returning to smoking. A ẓihār suspended on a condition takes effect when the condition occurs. So, if you return to smoking, the expiation for ẓihār becomes obligatory upon you, even if no one knows about your statement. The rule is that the knowledge of someone whose consent is not required is not a condition. The expiation for ẓihār is well-defined. This ruling applies if the utterance was spoken; however, if it was merely a thought, then nothing is incumbent upon it. Ibn Taymiyyah held the view that a suspended ẓihār intended for urging or preventing necessitates an expiation for an oath (kaffārat yamīn) upon breaking it, but the opinion of the majority of scholars is more cautious.