all of you must likewise pray sitting.’ So they all sat and completed the prayer. That is because the regular appointed imam is meant to be kept in front.

Should they, in that circumstance, pray behind him standing—or if some of them remain standing—their prayer is valid according to the sound position. Still, if the imam were to appoint someone else to lead them standing, that would be better: it avoids the disagreement, and the Prophet ﷺ himself once deputed another, having practiced both options to show their permissibility. Allah knows best.

The leadership of a person afflicted with a continuous state of minor impurity—such as chronic urinary incontinence, incessant flatulence, or any similar ongoing condition—is only valid for someone who shares that same affliction. A healthy worshipper’s prayer behind such an imam is invalid, for the imam’s prayer contains a flaw that has no compensatory substitute: he is praying while impurity is actually exiting, which contravenes the requirement of purity. His own prayer is deemed valid solely out of necessity, and likewise for someone in the same condition, since they are equal with respect to the uninterrupted discharge.

If someone prays behind an imam who is in a state of impurity—whether on his body, clothing, or the place of prayer—and neither is aware of that impurity or state of hadath until the prayer has been completed, the congregant’s prayer is sound whereas the imam’s is not. This is based on the Prophet’s ﷺ statement: ‘If a person in a state of major impurity leads the people, he must repeat his prayer…’