Question to Imaam Muhammad Ibn Saaleh Al-Uthaymeen (rahimahullaah):

Is it obligated on the Ma’moon (i.e. those being led in prayer by the Imaam) to rectify the Imaam if he makes a mistake in his recitation?

Answer:

If the Imaam makes a mistake in the ‘Obligatory Recitation’ such as the recitation of (Surah) Faatihah, it obligated on the  Ma’moon (i.e. those praying behind him) to disclose that to him (i.e. by rectifying him). If the recitation is Mustahab (i.e. recommended), we contemplate (on the following): If he changed the meaning, it is then obligated on him to repeat it [i.e. rectifies the mistake when prompted by the Ma’moon or if he (himself) realizes that he made a mistake that changes the meaning]. And if he has not changed the meaning, it is not obligatory (i.e. to rectify him).

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[Source: http://www.ajurry.com/vb/showthread.php?t=39055]