The ruling on one who stands for the third rak'ah out of forgetfulness and then sits

Question:
Honorable Shaykh, the questioner says: I was praying the rawatib after the midday prayer, so I stood for a third rak'ah out of forgetfulness. When I remembered, I sat down from standing directly into the tashahhud, without bowing or prostrating. What is the ruling on doing this?

Answer:
If you remember before you settle into standing, you must go back. You perform the first tashahhud, then prostrate for forgetfulness before the taslim or after the taslim. If you do not remember until after you have settled into standing, it is disliked to go back; you continue and prostrate for forgetfulness. If you do not remember until you have begun reciting al-Fatihah, it is forbidden to go back, because you have begun a pillar, and you do not return to an obligatory act by leaving a pillar. You leave the pillar you began in order to return for an obligatory act. This is the detail in this issue. Allah knows best, and may Allah send blessings and peace upon our Prophet Muhammad and upon his family and companions.