The Shaykh’s answer regarding a man who leads a group in prayer and then remembers that he was not in a state of ritual purity during the prayer: does his prayer become invalid alone, or does the prayer of everyone become invalid?

The Shaykh : As for your question about the imam who remembers during the prayer that he is not in a state of ritual purity, we say: it is obligatory for him to leave his prayer, and it is not permissible for him to continue, worshipping Allah in a manner not prescribed by Him and drawing near to Allah the Exalted by means that anger Allah. Indeed, Allah is not pleased with a servant who prays without ritual purity, or rather, without purity. He must leave and then say to one of the congregation behind him: “So-and-so, complete the prayer for them.” If he leaves without appointing someone to complete the prayer, the followers have a choice between two options:
Either they complete the prayer individually, or they appoint one of them to lead them in prayer.
As for their prayer, it is valid and there is no harm in it. There is no difference between remembering that he was not in a state of ritual purity and experiencing a state of ritual impurity during the prayer; there is no difference between these two cases. The follower is excused in any case, because the follower does not know about the imam’s state of ritual impurity, and if he had known that his imam was in a state of ritual impurity, he would not have prayed behind him.