Reference: al-Fataawa as-Sa’diyyah – Page 126, Question 11
Question: What is the ruling regarding praying behind one who smokes?
Response: If you are able to find another imaam, then do not pray behind him (the smoker). And if you were unable to find other than the imaam who smokes, then you are required to pray behind him; Do not pray alone by yourself. And Allaah [alone] has the Complete knowledge [of all affairs].