and that they are mistaken, he must not follow what they say. If he were to follow what they say, he would be doing so while knowing that they are mistaken, and his prayer would therefore be invalid.
Question: What if two unknown persons alert him by saying “Subhan Allah”? He should not follow what they say, because their trustworthiness has not been established. In reality, however, an imam may face this difficulty: he hears the glorification uttered behind him but does not know who made it. They may be trustworthy, or they may not be. Yet, in such a case, the imam will generally be in doubt and will consider it more likely that the two who alerted him are correct. He may then follow what they say, because the stronger view is that one should act on what appears most likely.
Question: If two women alert him by clapping—for example, if a man leads his mother and sister in prayer, makes a mistake, and they alert him by clapping—should he return, or not?
The answer is: he should return, for the Prophet, peace and blessings be upon him, said: “If something arises during the prayer, let the men say ‘Subhan Allah,’ and let the women clap.” (1) This is also because it is a religious report, in which males and females are equal; and because it is a report regarding an action in which they participate with the one performing it. They cannot falsely report it, for if he makes a mistake, they will have made the same mistake with him. Therefore, we say that two women are equivalent to two men.
The prayer of one who knowingly follows him—not out of ignorance or forgetfulness—is invalid, whereas the prayer of one who separates from him is not. ........
His statement: “The prayer of one who knowingly follows him—not out of ignorance or forgetfulness—is invalid, whereas the prayer of one who separates from him is not.”
That is: if two trustworthy persons alert the imam and he does not return, while not being certain that he himself is correct, his prayer is invalid, because he has failed to do what was obligatory upon him: returning to the correct course.