Is it obligatory to repeat a congregational prayer if the Imam forgot an ayah from Surah Al-Fatihah in the first rak'ah, taking into account the hadith: "There is no prayer for one who does not recite the Opening of the Book (Al-Fatihah)"?

Reciting Al-Fatihah is one of the pillars of prayer. If the Imam or a person praying alone forgets to recite it in a rak'ah, and then remembers after a long time, he must repeat the prayer. However, if he remembers after a short time, he should perform a rak'ah in its place and prostrate for forgetfulness (sujood as-sahw). If the Imam forgets a verse from Al-Fatihah and does not remember it until after a long period, repeating the prayer is obligatory. If he remembers before a long separation of time, he should perform a rak'ah in place of the one in which he omitted the verse and prostrate for forgetfulness. However, if the forgotten verse is from a surah other than Al-Fatihah, the prayer is valid, and there is nothing upon him. If the ma'mum (follower) forgets Al-Fatihah or is ignorant of it, he is not obligated to repeat the prayer; the Imam bears it on his behalf. In the case where the Imam forgets two verses from Al-Fatihah and no one prompts him, it is obligatory for both the Imam and the ma'mumeen to repeat the prayer, because the prayer is not valid unless Al-Fatihah is recited in its entirety.