The imam prayed with the people and, in the last rak'ah, told them he was not in a state of ritual purity, and one of them stepped forward and completed the prayer with them!

Question:
He says: Honorable Shaykh, may Allah grant you success. The questioner says: A group of young men prayed the 'Isha' prayer in congregation yesterday. During the last rak'ah, their imam turned to them and said, "I am not in a state of ritual purity." Then he left. One of the ma'mum stepped forward and completed the prayer with them. Their question is: Do they repeat their prayer, or is their prayer valid? They prayed behind the one who stepped forward and completed it with them.

Answer:
This is a point of difficulty and a matter of discussion. If he entered the prayer without wudu', the prayer is not valid, so they start it over from the beginning. But if he entered the prayer with wudu' and then his wudu' was broken during the prayer, he appoints a substitute to complete the prayer with them.