The ruling on the prayer of one who sat with 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 tasbih for him, but he did not return, while I remained sitting until he prayed the fifth and returned and I said the taslim with him, 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 had stood up for a fifth rak'ah, but in his case he had not stood up for a fifth; rather, he had stood up for a fourth because one of the rak'ahs was invalidated, as he left out a pillar of it. So you did not complete the prayer, and it is upon you to do this: if you say the taslim, you stand up and make up the rak'ah. If the congregation is informed and he tells them, "I stood up because I left out al-Fatihah and the rak'ah was invalidated," then you stand up, even after the prayer, even if a long time has passed, and you make up your rak'ah, because you prayed three rak'ahs.