Was what the worshiper did correct or not, when he returned to recite Al-Fatiha after doubting its validity in the third rak'ah of a four-rak'ah prayer, then completed his prayer and performed the prostration of forgetfulness?

If a worshiper doubts, after completing the Fatiha, whether he omitted one or more letters from it, he should not pay attention to this doubt, because the default is that it was completed. Also, doubts about its letters are frequent due to their large number, so they have been excused due to hardship. In this case, it suffices to rely on the predominant assumption, unlike other pillars of prayer. If the worshiper, after experiencing this doubt, ignorantly returns to the standing position, his prayer is valid due to his excuse of ignorance, and this addition does not invalidate his prayer.