The imam rising to the third rak'ah while forgetting the first tashahhud

Question:
Honorable Shaykh, may Allah grant you success. An imam rose to a third rak'ah while forgetting to sit for the first tashahhud, and some of the ma'mum (those praying behind the imam) rose with him. Then the imam began reciting, so some of the ma'mum reminded him, and he sat down. Some of them did not sit down. What should we do? And what is the correct ruling?

Answer:
The imam and the ma'mum who reminded him after he had started reciting are ignorant. One should not remind him after he has started reciting, because he has entered another pillar (rukn), so he does not return to sitting. He continues and performs the prostration for forgetfulness (sujud al-sahw). He continues and performs the prostration for forgetfulness in place of the first tashahhud. Yes.