What is the ruling on the prayer of a congregant who performs an action that invalidates it out of ignorance or forgetfulness? Is he required to repeat it if he is informed, and is the one who does not inform him sinful?

The prayer of a congregant who is not in a state of ritual purity is invalid and must be repeated. However, his prayer is not invalidated by an accidental cough. As for alerting the Imam verbally, by saying, for example, "You are not in a state of purity," this invalidates his prayer, and he must repeat it, because speaking during prayer without necessity invalidates it.

However, alerting the Imam without speech, such as by striking one's thigh, or by saying "SubhanAllah" (Glory be to Allah), or by clapping (with clapping being disliked for men and invalidating the prayer if excessive), does not invalidate the prayer.