The ruling on the prayer of the ma'mum (one praying behind the imam) with an imam who forgot al-Fatihah

Question:
May Allah reward you, Honorable Shaykh. This questioner says: The imam stood up for the fifth rak'ah, and we recited the tasbih for him, but he did not return. We had sat down in the fourth rak'ah and did not stand up with him. However, after he finished the fifth rak'ah and said the taslim, we said the taslim with him. He said that he forgot al-Fatihah in one of the rak'ahs and brought a different rak'ah. What is the ruling on our prayer with him in this case?

Answer:
It was obligatory for you, when he told you that he had not added anything and that he stood up only to bring a rak'ah in place of the rak'ah in which he left out al-Fatihah, it was obligatory for you to stand up and bring a rak'ah. As for the fact that you did not do so and are asking now, you must repeat the prayer from the beginning. You repeat the prayer from the beginning because your prayer is not valid, since you did not complete it when the imam told you that the rak'ah he brought is a substitute for the rak'ah in which he left out al-Fatihah.