Ruling on Deliberately Adding an Obligatory Act or Pillar to the Prayer

Question:
The sister questioner, who used the initials (W. W. W) for her name from Riyadh, says in this question: If one deliberately adds an obligatory act from the obligatory acts of the prayer, is his prayer invalid, like one who deliberately adds a pillar?

Answer:
This requires a detailed explanation:
If one deliberately adds a second bowing, or a third prostration, his prayer is invalid. But if he repeats the glorification, saying "Glory to my Lord, the Most Great" many times, or "Glory to my Lord, the Most High," that is fine; there is nothing wrong with it.
As for adding a pillar that is not prescribed to be repeated, or an obligatory act that is not prescribed to be repeated, the prayer is invalid. The first tashahhud is obligatory. So if he sits in the third rak'ah, or in the first rak'ah deliberately, and it is not the sitting for rest, but rather he considers it an extra sitting other than the rest, his prayer is invalid. Or if he adds a bowing, or adds a third prostration in the rak'ah, or a second bowing in the rak'ah, and he does so deliberately, his prayer is invalid. Yes.

The presenter: May Allah reward you, O Shaykh.