Smoking is one of the evil things that are forbidden, and it causes a great deal of harm. Allaah says in His Holy Book, in Soorat al-Maa’idah (interpretation of the meaning): 

“They ask you (O Muhammad) what is lawful for them (as food ). Say: ‘Lawful unto you are At‑Tayyibaat [all kinds of Halaal (lawful‑good) foods which Allaah has made lawful (meat of slaughtered eatable animals, milk products, fats, vegetables and fruits)]’” [al-Maa'idah 5:4]

 And He says in Soorat al-A’raaf, describing the Prophet Muhammad (peace and blessings of Allaah be upon him):

 “he allows them as lawful At‑Tayyibaat (i.e. all good and lawful as regards things, deeds, beliefs, persons and foods), and prohibits them as unlawful Al‑Khabaa’ith (i.e. all evil and unlawful as regards things, deeds, beliefs, persons and foods)”

[al-A’raaf 5:157] 

Undoubtedly smoking is an evil and unlawful thing, so your husband must give it up and keep away from it, in obedience to Allaah and His Messenger (peace and blessings of Allaah be upon him) and so as to avoid that which incurs the wrath of Allaah, and to protect his religious commitment and health, and to maintain good relations with you.  As expiation for his broken oath he must offer kafaarat yameen, as well as repenting to Allaah for starting to smoke again. This expiation means feeding or clothing ten poor persons, or freeing a believing slave; it is sufficient to give them dinner or lunch, or to give each of them half a saa’ of the staple food of the country Which is equivalent to a kilo and a half.

We advise you not to ask him for a divorce if he prays and he is a man of good conduct, and if he gives up smoking. But if he persists in this sin there is nothing to prevent you from asking him for a divorce.