What must the Imam and the congregants do if the Imam deliberately stands for a fifth Rak'ah in the Dhuhr prayer, claiming that he did not recite Al-Fatihah in one of the Rak'ahs?

If the Imam forgets the Fatiha in a rak'ah, that rak'ah is not counted, and he must perform another one, because the Fatiha is a pillar (rukn) according to the majority of scholars. As for the congregant (ma'mum), he should not follow the Imam if the latter stands for a fifth rak'ah. Instead, he should sit for the tashahhud and wait to make the salutation with the Imam, because the congregant's prayer is already complete.