What is the ruling on the prayer of an Imam who, when one of the congregants made him doubt during the final Tashahhud, stood up for a rak'ah, then prostrated, then stood up for another rak'ah and performed the prostrations of forgetfulness (sujud al-sahw), and it was later clarified that the congregant who made him doubt was mistaken?

If the congregants differ regarding alerting the Imam, and the Imam is not certain about the correctness of either group's statement, then he is obligated to build upon the lesser (number of units) and perform the prostration of forgetfulness (sujud (definition) al-sahw) before the salutation (tasleem). The congregants must follow him, except for the one who is certain of his error; he should wait until the Imam performs the salutation and then perform the salutation with him. The Shafi'is have adopted building upon certainty when doubting the number of rak'ahs, and this is the view of the majority of the Companions and Tabi'in, contrary to those who invalidated the prayer or acted upon the predominant assumption. The prayer of the Imam and the congregants is valid, except for the one who followed the Imam while being certain of his error; he must repeat the prayer.