What is the ruling on the follower continuing to pray behind the Imam who stood up for a fifth raka'ah and performed the prostration of forgetfulness (sujud al-sahw), knowing that the follower was aware it was the fifth raka'ah and sat waiting for him?

If the Imam stands for an extra rak'ah, and the congregant is certain of its excessiveness, then it is not permissible for him to follow the Imam. The Hanbalis are of the view that the congregant should separate from his Imam, recite the Tashahhud, and perform the Taslim, without waiting for his Imam's Taslim. Conversely, the Malikis are of the view that the congregant should glorify God (say "Subhanallah") to his Imam and sit until he performs the Taslim with him after the Imam completes the extra rak'ah.