What should the Imam do if he forgets to recite Al-Fatiha in one of the rak'ahs: Does he stand for a fifth rak'ah, and what is the situation of the congregants?

If the Imam remembers that he forgot to recite Al-Fatiha in the last rak'ah before the taslim (salutation) or shortly after it, he must return to the prayer. Based on what he did, he should then perform another rak'ah and make the prostration of forgetfulness (sujood as-sahw). If a long time has passed, he is required to restart the prayer. As for the congregants (ma'moomeen), they must follow the Imam unless they are certain that he added something to the prayer that does not belong. The prevalent opinion is that the recitation of Al-Fatiha is waived for the congregants as long as they are following an Imam.