Is it permissible to follow the imam if he adds to the prayer?
Machine translationRead the Arabic original
Question:
If you prayed four rak'ahs with the imam, and he made a mistake and stood up for a fifth rak'ah?
Answer:
You wait; you do not stand up with him. You wait until he sits down, then he says the taslim. You alert him by saying, "Subhan Allah, Subhan Allah." If he does not obey you, you and those with you who know that he added [a rak'ah] sit down. You do not stand up with him. When he finishes his prayer and says the taslim, you say the taslim.
If you prayed four rak'ahs with the imam, and he made a mistake and stood up for a fifth rak'ah?
Answer:
You wait; you do not stand up with him. You wait until he sits down, then he says the taslim. You alert him by saying, "Subhan Allah, Subhan Allah." If he does not obey you, you and those with you who know that he added [a rak'ah] sit down. You do not stand up with him. When he finishes his prayer and says the taslim, you say the taslim.
Source
binbaz.org.sa
The Arabic text is copied verbatim from the original source, without any edits.