Firstly: Reciting Al-Fatihah is an essential pillar (Rukn) of the prayer, and missing it must be rectified by actually reciting it according to the majority view. It is obligatory according to the Hanafi school, and its omission necessitates the prostration of forgetfulness. Each school has its own evidence for its stance.
Secondly: The fundamental principle in prayer is the sequential order of actions like standing, bowing, prostrating, and standing up again in between.
As for matters related to statements (recitations), the default is their recommendation (Sunnah) unless there’s explicit evidence specifying their manner, like in the case of audible and silent recitations.
Based on this, if someone forgets to recite Al-Fatihah and then remembers and recites it, there is no blame upon them according to the majority view, and their prayer is valid. This is because they fulfilled the pillar according to those who consider it a pillar, or fulfilled the obligation according to those who consider it obligatory.
The Hanbali school states that it’s recommended for the person who forgot to recite Al-Fatihah to perform the two prostrations of forgetfulness, based on the general statement of the Prophet, peace be upon him: “If any one of you forgets in their prayer, then they should perform two prostrations.”
And Allah knows best.
Fatwa by Dr. Khālid Naṣr