Does the prayer become invalid if one returns to the first tashahhud after standing up?

Question:
Honorable Shaykh, may Allah grant you success. This questioner says: One day I led the people in the Zuhr prayer and forgot the first tashahhud. After I had begun the third rak'ah, I remembered it, so I went back to the tashahhud. He says: One day I led the people in the Zuhr prayer and forgot the first tashahhud. After I had begun the third rak'ah, I remembered it, so I went back to the tashahhud. After I had finished the prayer, I was told that my prayer was invalid because I returned to the tashahhud after standing up for the rak'ah. Is my prayer valid or invalid?

Answer:
No, it is not invalid, but you made a mistake in this. If you relied on standing and began a pillar, you do not return from a pillar to an obligatory act, because the first tashahhud is an obligatory act. So you do not leave the pillar in which you had begun and return to an obligatory act; that is a mistake. Your prayer is valid, if Allah wills.