What is the ruling on a congregant following the Imam if he stands up for a fifth rak'ah in Dhuhr prayer?

The congregants must glorify God (say "Subhanallah") to the Imam if he adds an extra rak'ah. If two or more reliable congregants glorify God to him, he must return (correct his prayer) if his addition was due to doubt; otherwise, his prayer is void. It is not permissible for the congregants to follow him if they are certain that it is a fifth rak'ah. They may separate from him and say the taslim, or wait for him to say the taslim and then say it with him. The prayer of anyone who follows him intentionally, knowing it is forbidden, is void. The prayer of anyone who follows him out of forgetfulness or ignorance is valid.