I prayed the Fajr prayer, and in the first rak'ah I doubted whether I had performed the second prostration or not, then I stood up for the second rak'ah while I

Question:
Our sister says: I prayed the Fajr prayer, and in the first rak'ah I doubted whether I had performed the second prostration or not? Then I stood up for the second rak'ah while I was still in doubt, so I recited Surat al-Fatihah, then I went back and prostrated, then I stood up to perform the second rak'ah and completed it, then I prostrated for forgetfulness before the taslim and I said the taslim. What is the ruling on that, Honorable Shaykh? May Allah reward you with good.

Answer:
In the name of Allah, the Most Gracious, the Most Merciful. Praise be to Allah, Lord of the worlds, and may Allah send prayers and blessings upon our Prophet Muhammad, and upon his family and all his Companions. As for what follows, if a person leaves a pillar of a rak'ah of the prayer, or doubts whether he performed it, and stands up for the second rak'ah, then the rak'ah in which he left the pillar or in which he doubted is void, and the second rak'ah takes its place. He completes his prayer on that basis, and he prostrates for forgetfulness after the taslim or before the taslim.
As for what was mentioned in the question about the questioner going back after he had fully stood up to perform what he doubted about, this is a mistake. This is a mistake on his part. And if he built his prayer on this rak'ah, then his prayer is not valid, so he has to repeat the prayer; because he built it on a rak'ah that was invalidated by his standing up for the second rak'ah and beginning it. Yes.