Praise be to Allah, the Lord of the Worlds.
According to Sharia, smoking is Haram(Forbidden), and the same applies to selling and trading in cigarettes. It doesn`t befit an Imam to gain a reputation for committing such sin, taking into consideration his being in the lead of prayer performers. On the contrary, he should be keen on competing for doing all that is good and refraining from all that is evil.
Nevertheless, if the pillars and conditions of prayer are met, it is valid to pray behind such an Imam and there is no harm on praying people. Rather, the harm is on him since it is disliked for a person to lead people in prayer although he knows that they disapprove following him because of that sin.
Al-Khateeb Al-Shirbini (May Allah have mercy on him) said: "It is properly disapproved for an Imam to lead a people in prayer although he knows that they hate to follow him on account of his committing a disgraceful act. For example, an oppressive ruler, an Imam who imposes himself upon a mosque or doesn`t take heed from impurities, makes a living from unlawful money or keeps company with evil sinners. The proof of this is the Al-Khateeb Al-Shirbini (May Allah have mercy on him) said: "It is properly disapproved for an Imam to lead a people in prayer although he knows that they hate to follow him on account of his committing a disgraceful act. For example, an oppressive ruler, an Imam who imposes himself upon a mosque or doesn`t take heed from impurities, makes a living from unlawful money or keeps company with evil sinners. The proof of this is the narration of Ibn Majah: " It was narrated from Ibn ‘Abbas that the Messenger of Allah (PBUH) said: “There are three whose prayer do not rise more than a hand span above their heads: A man who leads people (in prayer) when they do not like him; a woman who has spent the night with her husband angry with her; and two brothers who have severed contact with one another.”{Moghni Al-Mohtajj, V.1/P.489}. And Allah knows best.