The ruling on changing the intention of standing in night prayer to witr when one stands for the third rak'ah out of forgetfulness

Question:
He says: Honorable Shaykh, I prayed the night prayer in two rak'ahs at a time. In one of the sets, after the second rak'ah, I forgot, and then I stood up for the third rak'ah while still in a state of forgetfulness. So I changed my intention to make that rak'ah witr, and I finished my night prayer. The question is: Is there any problem with changing my intention from the night prayer to witr?

Answer:
My brother, you made a mistake. It is obligatory for you, if you stand up for the third rak'ah in the night prayer, to sit down. It is obligatory for you to sit and recite the tashahhud until the end, then say the taslim, then bow and prostrate two prostrations for forgetfulness. This is what is obligatory for you, not to change, not to change the prayer to witr. Rather, it is obligatory for you to sit and recite the tashahhud, then say the taslim, and prostrate two prostrations for forgetfulness, whether you wish to do so before the taslim or after the taslim. Yes.