If the imam remembers that he is in a state of impurity after the tashahhud and before the taslim, should he say the taslim or what should he do, and do the congregants repeat the prayer or only the imam?

If the Imam remembers that he is in a state of impurity during the prayer, he must leave it immediately, even if it is just before the Taslim (the salutation of peace at the end of prayer). It is recommended for him to appoint one of the congregants to complete the prayer with the people. If he does not appoint anyone, it is permissible for the congregants to appoint someone to lead them, or they may complete their prayers individually, and their prayer is valid in all cases.