Ruling on praying behind someone who smokes cigarettes and the narghile
Question

                We have six classes each day, then we pray Zuhr and we have various people who lead us in prayer. Some of them smoke cigarettes, some smoke the narghile (“hubble-bubble”), and some grow their hair long. What is the ruling on these people leading the prayer? Is it permissible to pray behind them?
Praise be to Allah.Yes, 
          the prayer is valid, but it is better to appoint as your imaam the one 
          who reads the Qur’aan best, and who has the best understanding of Islam. 
          That is better, because the Prophet  
          (peace and blessings of Allaah be upon him) said: “Let the people appoint 
          as their imaam the one amongst them who reads the Qur’aan best.” (Narrated 
          by Muslim, no. 673). The meaning 
          of the one who reads the Qur’aan best is the one who reads it and acts 
          in accordance with it, because there is nothing good in reading it and 
          not acting in accordance with it. If the people appoint a person to 
          be the imaam, and among the congregation there is someone who reads 
          the Qur’aan better than he does, that is not right, as implied in the 
          hadeeth. 
 Imaam 
          Ahmad mentioned in his book Risaalat 
          al-Sunniyah: If a person leads people in prayer and among 
          them there is someone who is better than him, they will be in a state 
          of decline. So it is better for you to be led in prayer by the one who 
          is most pious, who has the best understanding of Islam and who has the 
          most knowledge of the Qur’aan. But if we suppose that this man who smokes 
          cigarettes or who shaves his beard or who smokes the narghile or who 
          grows his hair long comes forward and leads you in prayer, we say: the 
          prayer is valid, and you do not need to repeat it, because he is Muslim, 
          but the prayer is imperfect. 
 And 
          Allaah knows best.