What is the ruling on someone who mistakenly stood up for a third rak'ah during Tarawih prayers, intending only two rak'ahs, and is he required to perform a fourth?

For one who stands for a third rak'ah out of forgetfulness in Tarawih prayer or any other supererogatory prayer:

1. According to the Malikis: If he has not completed the bowing of the third rak'ah (i.e., he has not raised his head from its bowing): he returns and prostrates after the salutation, provided he has already lifted his hands and knees from the ground. Otherwise, there is no prostration due on him. If he has completed the bowing of the third rak'ah: he completes four rak'ahs (except for the two rak'ahs of Fajr) and prostrates before the salutation.

2. According to some scholars (including Sheikh Ibn Uthaymeen): He must return whenever he remembers and prostrates after the salutation (prostration of forgetfulness) because he has added to the prayer. If he does not return while being aware, his prayer is invalidated because the night prayer is two by two, and the worshipper's addition to that contravenes the command of the Prophet, peace and blessings be upon him.