What is the ruling on one who swore by Allah not to smoke, then smoked?

If one swears to abandon a sin, such as smoking, it becomes forbidden for him to break his oath. Breaking such an oath incurs sin from two perspectives: committing the forbidden act, and breaking the oath itself.

The one who breaks the oath must seek forgiveness from Allah, repent, and stay away from smoking. He also owes the expiation for his oath, as mentioned in the Qur’an: feeding ten poor persons from the average of what you feed your own families, or clothing them, or freeing a slave. But if he finds no means, then he must fast for three days.