Is repetition required after praying behind an Imam who is known to drink intoxicants?
Chapter on Imamate and Congregational Prayer
Al-Mughni
Book of Prayer
Primary text
One must not pray behind an Imam who drinks what causes intoxication, due to his open sinfulness. Ahmad explicitly mandated repetition behind one who drinks intoxicants. This ruling extends to every openly sinful person, as the legal consequence of intoxication is analogous to general immorality. If one prays behind such a person and later learns of this, one must repeat the prayer, and the repetition should be the prayer performed individually.
Supporting text
If the intoxicant consumed is one concerning which there is scholarly disagreement (like Nabidh) and the Imam believes it is permissible, then prayer behind him is acceptable, as Ahmad stated that one prays behind one who drinks intoxicants based on misunderstanding but not behind one who drinks them while knowing it is forbidden.