The ruling on the one who joins the prayer late and stands up before the prostration for forgetfulness with the imam

Question:
May Allah reward you, Honorable Shaykh. This questioner says: The imam made a mistake by sitting for the tashahhud in the third rak'ah of the 'Asr prayer, and I was among those who joined the prayer late. When he finished the prayer with the taslim, he performed the prostration for forgetfulness. As for us, we did not prostrate with him; rather, we stood up to complete the part of the prayer we had missed. Is our prayer valid?

Answer:
The prayer is valid, but you made a mistake by standing up. It is obligatory for you to prostrate with him, following him, and not to stand up until you have prostrated with him, following him. It is not as if you stand up and then the prayer is finished. However, your prayer is valid in view of your ignorance of this. The jurists say that if one has fully stood up before the imam prostrates, he does not go back. But if he has not fully stood up and the imam prostrates, he goes back and prostrates with him.