What is the response to one who claims that the fatwa prohibiting following the Imam when he stands for a fifth rak'ah is incorrect, citing the hadith: "The Imam is appointed to be followed..." and the hadith: "The Imam is a guarantor"?

Some Hanbalis held that it is obligatory to follow the Imam in an additional rak'ah, arguing that he might have forgotten a pillar, and that such a possibility does not invalidate the certainty of the obligation to follow by doubt. However, the correct position within the Hanbali school and the view of the majority is that it is forbidden to follow the Imam in what the follower knows to be an addition, because his action is considered void.

Therefore, if the Imam stands up for an additional rak'ah, the follower must alert him. If he does not return, the follower has the choice between waiting for him to conclude the prayer or leaving him and completing his prayer on his own. As for the Companions following the Prophet, peace be upon him, in the fifth rak'ah, it was out of ignorance or interpretation, and thus their prayer was not invalidated.

Sheikh al-Islam Ibn Taymiyyah was asked about an Imam who stood up for a fifth rak'ah and did not heed the glorification (tasbih). He replied that following out of ignorance does not invalidate the prayer, but with knowledge, it is not appropriate to follow; rather, they should wait for him to conclude the prayer with them or conclude before him, and waiting is preferable.

Those who argue for the obligation of following cite the hadith (definition), "The Imam is a guarantor," and the hadith, "Verily, the Imam is appointed to be followed." However, these hadiths do not indicate following the Imam in his error. "The Imam is a guarantor" means that he oversees the prayer and keeps count of its rak'ahs, and "to be followed" means following and not differing, not following in error.