The ruling on one who begins recitation after forgetting the first tashahhud

The questioner:
May Allah reward you with good. Umm Muhammad from the Eastern Region says: I was praying the 'Isha' prayer, and in the second rak'ah, instead of sitting for the first tashahhud, I said it and recited al-Fatihah. I was unable to return to sitting because I heard that it is not permissible for a worshipper to return to a pillar once he has begun it. What is the ruling in this case, and what is required of me?

The Shaykh:
Yes, it is as you heard. If one forgets the first tashahhud and stands up for the third rak'ah and begins recitation, it is not permissible for him to return, because he has begun a pillar, and one does not return to an obligatory act by leaving a pillar. One does not leave a pillar to return to an obligatory act. However, he compensates for this with sujud al-sahw (the prostration for forgetfulness), meaning he prostrates before the taslim, two prostrations for forgetfulness. Yes.

The questioner:
She says: If I forgot the prostration for forgetfulness in this case, is there anything required of me?

The Shaykh:
If you forgot and remembered shortly after, you prostrate for forgetfulness after the salam. But if you did not remember until a long time had passed, or your wudu' was broken, or you left the mosque, then the prostration for forgetfulness is dropped. Yes.