What must a congregant do if the imam concludes the prayer with a deficit of a rak'ah or a prostration after being alerted but does not respond? And should he recite the tashahhud and conclude the prayer with him despite knowing that the imam sat down in the third rak'ah of a four-rak'ah prayer?

If the worshiper (ma'mum) is certain that his imam has made a mistake by missing a rak'ah (unit of prayer) or a prostration (sajdah), or by sitting for the tashahhud in the wrong place, he must alert the imam. If the imam does not correct himself, the worshiper should intend to separate from him and complete his prayer individually. It is not permissible for the worshiper to follow his imam in a known error.