What must a smoker do after vowing to quit smoking, then only lasting one day before returning to smoking?

Refraining from smoking is obligatory. The majority of scholars hold that a vow (nadhr) is not valid for something already obligatory because a vow is an obligation one places upon oneself for an act of devotion that is not already binding, whereas an obligatory act is already binding. However, Sheikh al-Islam Ibn Taymiyyah is of the opinion that expiation (kaffarah) is required, as these contracts and covenants necessitate a second obligation. Abandoning them entails abandoning what is obligatory both by divine law and by the vow, and this is more cautious and more absolving of one's responsibility.