Does divorce take effect if a man swears by divorce to quit smoking, but is unable to do so, and what should he do if he has had intercourse with his wife thereafter, believing that the divorce had not taken effect?

Divorce occurs by breaking an oath sworn by divorce. If the divorce was not the third (and final) one, and the wife was taken back before the expiry of her 'iddah (waiting period), then she has returned to his marital bond through intercourse. Sheikh al-Islam Ibn Taymiyyah holds the view that swearing an oath by divorce with the intention of emphasizing something or preventing an action does not effect a divorce; rather, it necessitates an expiation for an oath.

The legislated oath is to swear by Allah the Almighty. As for swearing by divorce, it is among the oaths of the dissolute and may lead to grave consequences.