What is the ruling on forgetting to recite Al-Fatihah in prayer, for the Imam or for the congregant?

Reciting Al-Fatihah is a pillar of prayer, without which the prayer is invalid. It must be recited by the individual worshipper, the Imam, and the congregant.

If the Imam forgets Al-Fatihah in the first unit (rak'ah) and only remembers in the second, then the second unit becomes the first, and he must perform an additional unit. As for the congregant, he does not follow the Imam in this unit; rather, he sits for the Tashahhud and performs the salutation (taslim) with the Imam.

If the congregant omits Al-Fatihah, then according to those who do not consider it obligatory for him, there is no consequence. However, according to those who consider it a pillar for him, he must perform an additional unit after his Imam's salutation, unless he joins the Imam while he is bowing (rak'i') or joins him while standing and bows before completing its recitation, in which case it is waived for him.