These are two issues:

The first issue: Prayer behind one who is in a state of ritual impurity is valid on condition that both the imam and the follower remain unaware of this until the prayer is completed.

An example involving minor ritual impurity is that an imam eats camel meat without knowing that it is camel meat, and then leads the congregation in prayer while they do not know that he has eaten it. Once the prayer has ended, he learns that the meat he ate was camel meat. In this case, the followers do not repeat their prayer, whereas the imam must repeat it. As for the imam, this is because he prayed without ablution, and the Prophet, peace and blessings be upon him, said: “Allah does not accept the prayer of any of you if he breaks his ablution until he performs ablution.” (1)

As for the follower, his excuse is evident: he does not know the unseen, and Allah does not burden any soul beyond its capacity.

But if he comes to know, during the prayer, that he is in a state of ritual impurity, then his prayer is invalid. What is meant is that it becomes clear that it was never validly commenced. The prayer of the followers is likewise invalid.

The invalidity of his own prayer is evident, since it becomes clear that he was without ablution, and thus that his prayer was never validly commenced.

As for the prayer of the followers, it is because it becomes clear that they were following one whose prayer was invalid; consequently, their prayer is invalid, since their prayer is contingent upon the prayer of their imam. Thus, when the imam’s prayer is invalid, the follower’s prayer is invalid.

If both he and the follower remain unaware until it has ended, it is valid for the follower alone.

If one of the followers comes to know, while the rest remain unaware—neither the imam nor the remaining followers knowing—then all of their prayers are invalid, due to the author’s statement: