The ruling on one who forgets the first tashahhud

Your Eminence, the questioner (Dr. A) from al-Ahsa asks: If a person makes a mistake in his prayer and stands up from the second rak'ah before reciting the tashahhud, and he does not remember until he is in the third rak'ah, does he go back and sit for the tashahhud? And if he remembers before he straightens up standing, does he go back and recite the tashahhud? In both cases, does he perform the prostration for forgetfulness?
Answer: If he stands up from the first tashahhud and remembers while he is in the process of standing, he must go back, recite the tashahhud, and perform the prostration for forgetfulness, as the Prophet (peace and blessings be upon him) did. He stood up from the first tashahhud and did not go back, but he prostrated twice for forgetfulness before the taslim. There is no harm in that.
As for if he has begun the recitation and does not remember until after he has started the recitation, he does not go back. He continues and performs the prostration for forgetfulness, and praise be to Allah, before the taslim. Yes.