When does one prostrate for forgetfulness if one leaves an obligatory act of the prayer?

Question:
May Allah reward you, Your Eminence, father. The questioner says: If a person praying leaves the first tashahhud out of forgetfulness, and a long time passes after the completion of the prayer, and then he becomes certain that he did not perform the tashahhud, does he repeat the prayer, which is obligatory? And if he is asked to repeat it, is there evidence for the obligation of repeating it upon him?

Answer:
The prostration for leaving an obligatory act: if he remembers it during the prayer, he prostrates before the taslim. If he remembers it shortly after the taslim, he also prostrates. However, if a long time has passed, the prostration for forgetfulness is dropped in this case, and his prayer is valid, if Allah wills.