The ruling on one who enters the prayer intending it to be a sunnah, then completes it as an obligatory prayer

This is a message we received from the questioner: Jaamil 'Abd al-Wahhab from Yemen - Taiz - Hawd al-Ashraf. He says: I intended to pray the Zuhr prayer, but when I said the takbir, I intended it to be a sunnah. When I reached the tashahhud, I forgot to say the taslim, so I stood up and completed the prayer. I did not remember that I had intended it to be a sunnah until after I had completed the prayer. Is my prayer valid or not?
Answer: It is not valid as an obligatory prayer, but it is a voluntary prayer. It remains entirely a voluntary prayer based on the first intention. You must perform the obligatory prayer afterwards, because actions are by intentions, and you intended it to be voluntary. It does not become an obligatory prayer; rather, it remains in its state as a voluntary prayer. You must perform the obligatory prayer. Yes.
The presenter: May Allah reward you.