What is the ruling on saying: "If I return to smoking, then my wife is to me as the back of my mother"?

Your pronouncement of Dhihar to your wife, contingent on your return to smoking, means that if you do smoke again, you will have broken your oath, and the Dhihar will take effect. You must then offer expiation (kaffarah) before you may touch your wife. The expiation is: freeing a slave; if you cannot find one, then fasting for two consecutive months; if you are unable to do that, then feeding sixty poor people. It is not permissible for you to approach your wife before performing this expiation. These three types of expiation must be performed in this specific order.