If the imam or a person praying in congregation forgets to recite al-Faatihah, what should he do?
Question

                What is the ruling if an imam or a person praying in congregation forgets to recite al-Faatihah during the prayer?.
Praise be to Allah.
    Firstly: we have already stated in the answer to question no
    
    10995 that 
    reciting al-Faatihah is an essential part of the prayer without which the 
    prayer is not valid. It is essential to recite it, whether one is praying 
    alone, as an imam or behind an imam, and whether the prayer is one in which 
    Qur’aan is recited out loud or quietly. 

    Secondly: If the imam forgets it in the first rak’ah, and he 
    does not remember until he stands up for the second rak'ah, then the second 
    rak’ah becomes the first rak'ah for him. Based on this, he has to offer 
    another rak’ah to make up for the first rak’ah in which he did not recite 
    al-Faatihah. With regard to the people praying behind him, they should not 
    follow him in this rak’ah, rather they should sit for the tashahhud, then 
    wait and say the salaam with the imam. 

    With regard to a person praying behind an imam who forgot to 
    recite al-Faatihah, for those who say that the person who prays behind an 
    imam does not have to recite al-Faatihah the matter is straightforward: he 
    does not have to do anything. For those who say that it is an essential part 
    of the prayer for him, and he is like the imam, then if he did not recite 
    it, he has to offer another rak’ah after the imam says the salaam, unless he 
    joined the prayer when the imam was bowing, or when the imam was standing 
    but he bowed before he could finish reciting it. In that case he does not 
    have to recite it in the first rak’ah.

    Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) ،
    Liqaa’aat al-Baab il-Maftooh, 51.