Praise be to Allaah.
Smoking is haraam, and there is no doubt about that among the scholars. It is harmful and there is no doubt about that among doctors and those who are wise. If the imam of a mosque does this deed, this is regarded as committing sin openly, and some scholars say that people should not pray behind him; the correct view is that it is permissible, but someone else who is of good character is better.
Shaykh ‘Abd al-‘Azeez ibn Baaz (may Allaah have mercy on him) was asked:
What is the ruling on smoking, and the ruling on people who do that openly leading prayers?
The shar’i evidence indicates that smoking is something which is forbidden, because it involves many evils and harmful effects, and Allaah has only permitted to His slaves food and drink which are good and beneficial. As for that which will harm their spiritual or worldly interests or alter their minds, Allaah has forbidden it to them, and He is more merciful towards them than their own selves, and He is All-Wise and All-Knowing in His words, deeds, laws and decrees. He does not forbid anything in jest or create anything in vain, and He does not enjoin anything that is not of benefit to His slaves, because He is the wisest of the wise and the most Merciful of those who show mercy, and He knows what is in the best interests of His slaves and will benefit them in this world and in the Hereafter, as He says (interpretation of the meaning):
“Certainly your Lord is All‑Wise, All‑Knowing”
“Verily, Allaah is Ever All‑Knowing, All‑Wise”