“If both he and the follower remain unaware until it has ended, it is valid for the follower alone.” That is, provided that none of the followers knows that he is without ablution. If even one of them comes to know—even during the prayer—the prayer of them all is invalid. There is no clear rationale for this second ruling: namely, that if one of the followers comes to know, everyone must repeat the prayer. As for the first ruling, it has a rationale that was mentioned previously.
An example involving major ritual impurity is that a man wakes from sleep, performs ablution, and goes to lead the prayer. After finishing the prayer, he sees on himself traces of major ritual impurity, though he had been unaware of it. In this case, we say: the followers’ prayer is valid.
As for him, he must repeat the prayer. But if he, or one of the followers, comes to know during the prayer, then the prayer is invalid.
The sound view in this matter is that the followers’ prayer is valid in every case, except for anyone who knows that the imam is in a state of ritual impurity.
This is because they were unaware and are therefore excused on account of ignorance. It is neither within their ability nor required of them to ask their imam: “Are you in a state of ablution or not? Are you in a state of major ritual impurity or not?” If they are not required to do this, and he leads them in prayer while knowing that he is in a state of ritual impurity, then how can their prayer be invalid?!
Here there is a very important principle: “Whoever does something correctly in accordance with the evidence of the Sacred Law, it cannot be invalidated except by evidence from the Sacred Law.” For if we were to invalidate that whose validity has been established by evidence, this would amount to speaking about the Sacred Law without knowledge, and to imposing hardship and difficulty upon those legally accountable. They did what they were commanded to do in following this imam; and whatever they were not charged with does not entail any ruling upon them.