What is the ruling on one who says to his wife, "If you do this habit again, then my wife is unlawful to me," intending thereby to quit a bad habit, but then reverts to it? And is an expiation (kaffarah) required of him? If so, what is it?

The preponderant opinion regarding the statement "My wife is unlawful to me" is to refer to the intention of the speaker: whether he intended divorce, ẓihār, or an oath. Since you did not intend divorce or ẓihār, the ruling on this comment is that of an oath, and you are liable for expiation for breaking it. This expiation is feeding ten poor people or clothing them, or fasting three days if you cannot find the means. It is necessary to draw attention to the prohibition of swearing by other than Allah, and that using oaths to refrain from sins is not a sound approach.