An imam stood up for the third rak'ah and forgot the tashahhud, then remembered and went back to sit. What is the ruling on his prayer?

Question:
He says: Honorable Shaykh, may Allah grant you success. If an imam, when it is time to sit for the second rak'ah for the first tashahhud, does not sit? If an imam, when it is time to sit for the second rak'ah for the first tashahhud, does not sit, but instead straightens up for the third rak'ah, and after he straightens up, the ma'mum (those praying behind him) alert him, so he goes back and sits, and at the end of the prayer he performs sujud al-sahw (the prostration for forgetfulness), is his prayer valid, given that he prioritized an obligatory act over a pillar?

Answer:
There is a distinction here. If he had not started the recitation, it is disliked (makruh) for him to go back; it is disliked, but his prayer is valid, and he must perform sujud al-sahw because he added to the prayer. However, if he had started the recitation, it is forbidden (haram) for him to go back because he had started the pillar. He had started reciting al-Fatihah, which is starting the pillar. So he does not leave it to go back to an obligatory act, which is sitting for the tashahhud. Doing so would be haram. As for the invalidity of his prayer, if he was ignorant of this matter, his prayer is not invalidated; he is excused due to ignorance, but he must perform sujud al-sahw.