Is it permissible to follow the imam if he adds a rak'ah to the prayer?

Question:
Now, Shaykh Salih (may Allah preserve you), what is the state of the ma'mum (one praying behind the imam) when the imam adds rak'ahs?

Answer:
If he knows that they are extra, it is not permissible for him to follow him. Rather, he should sit down. If he wishes, he may say the taslim (closing salam) and then perform the final tashahhud (testification). Then, if he wishes, he may say the taslim for himself, or if he wishes, he may wait and say the taslim with the imam. Yes.