What is the ruling on praying behind an Imam when you are certain that his ablution is invalid?

If the worshiper (ma'mum) knows that his imam is not in a state of ablution (wudu') and yet follows him in prayer, then his prayer is invalid. There is disagreement regarding the invalidity of the prayer of the other worshipers who are unaware of this. Some have invalidated the prayer of everyone, while others have restricted the invalidity only to the one who knew about the ritual impurity (hadath). The latter opinion is the correct one, due to their excuse of ignorance.