Should a traveler perform the prostration of forgetfulness (sujood al-sahw) if he prays 'Isha and stands for the third rak'ah, then completes it?

If a worshipper completes a third rak'ah out of forgetfulness, he should perform two prostrations (sujood) after the salutation (tasleem). However, if he completes it knowing that it is an extra rak'ah, his prayer becomes invalid, and he must repeat it. He is also required to sit down immediately upon realizing the addition.