Should I perform the prostration of forgetfulness (sujud al-sahw) after the Imam added an extra rak'ah in the 'Isha prayer, and I had already missed one rak'ah?

If the congregant is certain that his Imam stood for a fifth rak'ah and followed him, his prayer becomes invalid.

A congregant who joined late (masbuq) may, if his Imam stands for a fifth rak'ah, stand to complete his prayer intending to separate, or he may sit until his Imam finishes the prayer (makes salam), and then stand to complete what he missed.

The majority of scholars hold that the congregant follows his Imam in the prostration of forgetfulness (sujud al-sahw) if he caught any rukn (pillar) with him before the prostration of forgetfulness, whether it was before the salam or after it.

The Malikis hold that if the congregant did not catch a rak'ah with the Imam, he is not required to perform the prostration of forgetfulness, and if he prostrated with him, whether intentionally or out of ignorance, his prayer becomes invalid. And if the Imam prostrated after the salam and the congregant prostrated with him, his prayer becomes invalid, even if he caught a rak'ah.