What is the ruling if the imam makes a mistake in prayer but insists that he is correct?

Question:
I prayed behind an imam in a four-rak'ah prayer, and he prayed three rak'ahs with us. When we recited the tasbih for him, he did not respond and insisted on his action. After the taslim, he insisted, saying that he had prayed four rak'ahs. As for me, I am certain that he did not pray more than three. Do I suffice with what I prayed with him, or do I repeat the prayer?

Answer:
If the imam says the taslim while missing a rak'ah in a four-rak'ah prayer or any other prayer, and you know that he is deficient, you should alert him by saying, "Subhan Allah, Subhan Allah." He is acting based on his ijtihad, doing what he has: if he is certain that you are mistaken and that he did not make a mistake, then he follows what he believes. If he sees that it is not an extra rak'ah, he does so, and if he sees that it is not a missing rak'ah, he does so. And you must act according to what you believe: if you believe that it is deficient, you stand up when he says the taslim and you complete the rak'ah. If a long interval has passed, you repeat the entire prayer. As for him, his prayer is valid if he believes that. Likewise, the prayer of those with him is valid if they agree with him, or if they have no knowledge; they have no knowledge of the addition or the deficiency, so their prayer is valid following their imam. As for you, who believe that it is deficient, you stand up and complete it when he says the taslim. If you leave that and a long interval has passed, you repeat it from the beginning in its entirety.