The ruling on the prayer of one who followed an imam who stood up for a fifth rak'ah

Question:
He says: Honorable Shaykh, may Allah grant you success. If I prayed behind an imam who stood up for a fifth rak'ah, and I recited the tasbih for him but he did not straighten his prayer, while I remained sitting until he prayed the fifth and returned and said the taslim with me, and it turned out he had forgotten al-Fatihah in one of the rak'ahs and brought something else in its place, what is the ruling on my prayer in this situation?

Answer:
You thought he stood up for a guard. As for him, he thought he had organized his standing for the fourth, or rather he stood up for the fourth because one of the rak'ahs was deficient, because he left out a pillar of it. If you did not complete the prayer, and it is more likely that you did not, then when he says the taslim, you stand up and make up the rak'ah. If the congregation is informed, and he says to him, "I stood up because I read al-Fatihah and missed a rak'ah," then you stand up, even after the prayer, even if a long time has passed, and you perform a rak'ah, because I prayed and said the taslim for a rak'ah.