What is the ruling on one who stood for a fifth rak'ah in which the imam made a mistake?

Question:
One who prayed five rak'ahs with the imam?

Answer:
If he has no knowledge, he follows the imam and nothing is required of him.
Q: Does he repeat his prayer?
A: No, he does not repeat it, as long as he followed the imam, nothing is required of him. The imam prostrates for forgetfulness if he has doubt, and if he has no doubt, the basis is what he believed.
Q: And one who prayed with him, knowing it was five, and knowing he stood for the fifth?
A: Is he ignorant?
Q: No, he is not ignorant, but he followed.
A: And he is not ignorant?! Only an ignorant person stands up; otherwise, if he is not ignorant, he would sit and alert them, saying: Glory be to Allah, Glory be to Allah.
Q: Does he repeat his prayer?
A: It is not possible for him to be knowledgeable, except that a person is ignorant or forgetful. The knowledgeable person does not stand up; the knowledgeable person sits until the imam says the taslim, then he says the taslim with him, and prostrates for forgetfulness with him.