What do the followers do if the imam leaves them in the middle of the prayer

Q: If the imam steps forward and prays part of the prayer, then remembers that he had not performed wudu' and leaves, do the followers build upon what has passed or do they restart the prayer?

A: The correct view is that he should appoint a successor to lead them in prayer, as 'Umar (may Allah be pleased with him) did when he was stabbed. If he did not appoint a successor and they appointed one who completed the prayer for them, their prayer is valid. This is the stronger position among the scholars' views. If they prayed individually and completed the prayer for themselves, there is no harm.
As for if he had completed the prayer with them and then remembered that he had prayed without wudu', their prayer is valid and he repeats it for himself.