Shaikh Abdul-Muhsin al-‘Abbad hafidhuhallah was asked:
What is the ruling on the prayer behind an Imam who memorized the Quran, however, he smokes marijuana.
The Quran does not benefit this one. If it benefitted him, he would not be smoking this filth. The likes of this one is not fit to be the Imam nor should he be put forward. Someone else is put forward. Him having memorized the Quran and doing this evil, detestable action and crime makes him not fit to be the Imam. Yes”.
[Sharh Ibn Majah no. 76]
Faisal Ibn Abdul Qaadir Ibn Hassan