Aslam

Praise be to Allah.This

is known as the issue of “correcting the imaam”.

Correcting

the imaam is divided into two categories:

That which is obligatory, which is correcting the

imaam concerning something which would invalidate the prayer if done

deliberately. If he makes a grammatical mistake which changes the meaning

of al-Faatihah, then he must be corrected, because this type of mistake

which changes the meaning of  al-Faatihah invalidates the prayer.

Similarly, if he omits a verse of al-Faatihah, he must be corrected,

because that invalidates the prayer. The kind of correction which is

mustahabb (recommended) is when the imaam makes a mistake which does

not invalidate the prayer altogether, but it does undermine the perfection

of the prayer. So if the imaam forgets to recite another soorah after

al-Faatihah, alerting him to this fact is sunnah.

The

evidence for that is the words of the Prophet

(peace and blessings of Allaah be upon him): “I am a human being like

you, I forget as you forget. If I forget, then remind me.” (narrated

by al-Bukhaari, al-Salaah, 401). So he commanded them to remind

him. On one occasion when he became hesitant in his recitation, he said

to Ubayy ibn Ka’b, “What stopped you?”, i.e., what stopped you from

correcting me? This indicates that correcting the imaam is something

that is essential.

See

al-Sharh al-Mumti’ by Ibn ‘Uthaymeen, part 3, p. 346-347