What is the ruling on the prayer of someone who reminds the Imam in case of forgetfulness, and what is the extent of the obligation upon the worshipper to remind the Imam if he forgets during the first Tashahhud? Should the Imam complete his salutation and then say the Takbir intending two additional rak'ahs to mend the prayer, and then perform the prostration of forgetfulness?

It is recommended for the congregants to alert the Imam if he makes a mistake in his prayer, whether he forgets a unit (rak'ah) or a prostration (sajdah), or errs in recitation. This is done by saying "Subhanallah" (Glory be to Allah) for men, and by clapping for women, according to the Shafi'i, Hanbali, and Hanafi schools of thought. It is sufficient for one worshiper to give the alert, and there is no harm in repeating the tasbeeh by more than one person or by the same person, and it does not invalidate the prayer.