What is the ruling regarding someone who made a covenant with Allah to quit smoking, then recovered and is able to quit, but with difficulty? Is it permissible for him to expiate his covenant?

Our fatwa concerning the statement "I make a covenant with Allah that I will do such-and-such" is that it is either an oath or a vow, provided the speaker commits to it as an act of worship. Smoking is forbidden, and quitting it is obligatory and an act of obedience. Therefore, "a covenant with Allah to quit smoking" is considered a vow, specifically a vow to perform an obligatory act. The majority of scholars hold that a vow to perform an obligatory act is not binding, because a vow involves a person obligating themselves to an act of worship that is not already legally binding. An obligatory act, however, is already legally binding, and obligating oneself to something that is already legally binding is redundant. Based on this, the aforementioned vow is not binding, and failing to fulfill it does not incur expiation. Rather, repentance from smoking is required.