The ruling on the prostration for forgetfulness for one who forgets in a voluntary prayer

Our brother has another question regarding the tarawih prayer. His Eminence the Shaykh says: We request clarification of the ruling; if the imam prays with us three rak'ahs in tarawih and concludes the prayer assuming it was two rak'ahs, is there a prostration for forgetfulness in the Sunnah, or not?
Answer: Yes, the prostration is required. If he intended two rak'ahs as is customary, meaning: the Sunnah for night prayer is in pairs, then if he stands up for a third, he should be alerted by saying: Subhan Allah, Subhan Allah. He must return and complete the recitation of the tashahhud, the prayer upon the Prophet (peace and blessings be upon him), and the supplication. Then he performs the prostration for forgetfulness, and then concludes the prayer. If he proceeds and does not sit, they should alert him to sit, even after the third rak'ah. He sits and recites the tashahhud and what follows it, namely the prayer upon the Prophet (peace and blessings be upon him) and the supplication, then he performs the prostration for forgetfulness. Just as it is prescribed in the obligatory prayer, it is prescribed in the voluntary prayer. Yes.