Ruling on following the imam in the fifth rak'ah

Question:
In the four-rak'ah prayer, the imam stood up for the fifth rak'ah by mistake. Some of the worshippers followed him, while others waited until he completed the fifth rak'ah and said the taslim. Which of these actions is correct?

Answer:
All of their prayers are valid. The one who stood up out of ignorance has a valid prayer, and those who knew it was an extra rak'ah and waited until the imam said the taslim have a valid prayer, and that is what is obligatory for them. If the ma'mum knows it is an extra rak'ah, he should not stand up, but rather sit and wait until the imam says the taslim, and he should alert the one who is unaware to stand up so he can follow the imam. This is because the principle is to follow the imam, and all of their prayers are valid, if Allah wills.