What is the ruling on the prayer of the congregants if the Imam informs them after he has finished the prayer that he was without ablution due to forgetfulness?

The prayer of the congregants is valid if it becomes clear that the Imam was not in a state of ritual purity. However, the Imam's prayer is invalid, and he must repeat it. This is the more preponderant of the two opinions among scholars. This has been narrated from Umar, Uthman, Ali, and Ibn Umar, may Allah be pleased with them, and it is also the view of Malik and Al-Shafi'i.