What is the ruling on the prayer of an imam who remembered, while reciting in the first rak'ah, that he had performed istinja' but had not performed wudu', and what should he do in this situation?

If the Imam remembers that he is without ablution (wudu'), or remembers an impurity, or bleeds from the nose, he must withdraw immediately. It is sunnah for him to appoint one of the congregants to complete the prayer with them. It is not permissible for him to continue the prayer while in a state of ritual impurity (hadath). If he does not appoint a successor, the people may pray individually or appoint one of them to complete the congregational prayer.