Ruling on the prayer of one who says “Allahu akbar” when rising from bowing
Question

                I prayed behind a brother from India, who does not have much knowledge. He said “Allahu akbar (Allah is most great)” when rising from bowing, and I said “Sami‘a Allahu liman hamidah (Allah hears those who praise Him)” to myself. Do I have to repeat the prayer? When I spoke to him, he said that he did not know.
Praise be to Allah.
    Firstly: 

    Saying “Sami‘a Allahu liman hamidah” when 
    rising from bowing is one of the obligatory parts of prayer according to the 
    correct opinion. For more details, please see the answer to questions no.
    65847 and 90094. 

    Secondly: 

    Once this is understood, if a person omits 
    one of the obligatory parts of the prayer – which includes saying “Sami‘a 
    Allahu liman hamidah” – his prayer is not rendered invalid unless he did 
    that deliberately. If he omitted it because he forgot or was ignorant of the 
    ruling, then his prayer, and the prayer of those who prayed behind him, is 
    valid. But he has to do the prostration of forgetfulness (sujood as-sahw) 
    when he remembers it or learns of the ruling, if not much time has passed. 
    If a long time has passed, he does not have to do it, because the time for 
    it has gone. 

    It says in Daqaa’iq Ooli an-Nuha 
    (1/219): Whoever omits an obligatory part (of the prayer) because he is 
    ignorant of the ruling, in the sense that it never crossed his mind that a 
    scholar would say it was obligatory, is like the one who forgot or made a 
    mistake. So he should do the prostration of forgetfulness if he learns of 
    that before the time ends, otherwise there is no need, and his prayer is 
    still valid. End quote. 

    Shaykh ‘Abd al-‘Azeez (may Allah have 
    mercy on him) was asked: 

    What should the imam or person who prayed 
    behind him do if he says “Allahu akbar” instead of “Sami‘a Allahu liman 
    hamidah” or vice versa? 

    He replied: 

    It is not permissible for him to do that 
    deliberately; rather what the worshipper must do is pray as the Prophet 
    (blessings and peace of Allah be upon him) did. So he should say “Allahu 
    akbar” when appropriate and “Sami‘a Allahu liman hamidah” when appropriate, 
    and “Rabbana wa laka al-hamd (Our Lord to You be praise)” when appropriate. 
    Whoever does something other than that by mistake, there is no sin on him, 
    but he should do the prostration of forgetfulness, if he was leading the 
    prayer or praying on his own.

    End quote from Majmoo‘ al-Fataawa 
    (29/286) 

    Thirdly: 

    There is no blame on you for having said 
    both “Sami‘a Allahu liman hamidah” and “Rabbana wa laka al-hamd.” The 
    Shaafa‘is stated that it is mustahabb, i.e., it is mustahabb for the one who 
    is praying behind an imam to say both “Sami‘a Allahu liman hamidah” and 
    “Rabbana wa laka al-hamd,” although the more correct scholarly view is that 
    the one who is praying behind an imam should say only “Rabbana wa laka al-hamd.” 
    We have discussed the difference of scholarly opinion previously, in the 
    answer to question no. 43574 

    To sum up: The prayer of your imam is 
    valid, because he did not deliberately omit to say “Sami‘a Allahu liman 
    hamidah” and “Rabbana wa laka al-hamd.” Rather he omitted it because of his 
    ignorance. May Allah reward you with good for alerting him to that. 

    And Allah knows best.