The Ruling on the Imamate of One Who smokes

Question :

Is it permissible to pray behind an Imam who smokes?

Answer:

Smoking is forbidden, because it has been confirmed that it is harmful to the health, it is from Al-Khab'ith, and because it is a waste of money. Allah, the Most High says:
And he prohibits them as unlawful Al-Khaba'ith (i.e. all evil and unlawful things as regards deeds, beliefs, persons foods etc.).

As for the ruling on praying behind him, if avoiding behind him entails missing the Friday prayer or the congregational prayers, or results in some turmoil, it is obligatory to pray behind him, so as to do what is less harmful in preference to what is more harmful. But if there is no fear that some people's avoidance of praying behind him will cause them to miss the Friday prayer or the congregational prayers, nor any harm, and not praying behind him will only cause him to be rebuked and stop smoking, then avoiding prayer behind him is lawful, so as to deter him from doing what Allah has forbidden - this is in order to censure evil.

If avoiding prayer behind him does not involve any harm, nor cause others to miss the Friday prayer or the congregational prayers, yet he (the Imam) remains unchecked by the abandonment of prayer behind him, then the Muslims should look for one who, unlike him, is not guilty of sin and disobedience. This is more perfect for his prayer and safer for his religion. And through Allah is attained success. And may peace be upon Muhammad and upon his family and Companions.

Source:
The Permanent Committee
Fatawa Islamiyah, Vol. 2 Pages 400-401