The imam recited a verse where a prostration is required and did not prostrate, but the person praying behind him prostrated and missed the rukoo’ (bowing)
Question

                At ‘Isha’ prayer yesterday I entered the mosque when the imam was reciting Soorat al-‘Alaq in the second rak’ah and I joined the prayer. At the end of the soorah there is a verse where a prostration is required, but the imam said takbeer after this verse. Some of the worshippers bowed but most of them prostrated. I prostrated with them because I could not see the imam. After that the imam stood up again (by saying Sami’a Allaahu liman hamidah) and we realized that we had made a mistake and we completed the prayer with the imam, until he said the salaam. After that I made up the rest of the prayer and I did two rak’ahs instead of one, and I did the prostration of forgetfulness. Is what I did permissible, or should I repeat my prayer?.
Praise be to Allah.
    If the matter is as described, then your prayer is valid, 
    praise be to Allaah. 

    The reason for that is that the one who misses bowing with 
    the imam, for a reason like the confusion that you mention, has to do it and 
    then follow the imam, and in that case he is forgiven for his delay in 
    following the imam. If he does not do it, then the rak’ah is invalid, and he 
    has to do it again after the imam says the salaam, and you did that. 
It says in 
    Kashshaaf al-Qinaa’ (1/466): If the person praying behind the imam 
    misses an essential part of the prayer for a reason such as sleeping or 
    confusion or because the imam moves too quickly, he should do it and catch 
    up because that is obligatory, because he can catch up with him without any 
    problem, so he has to do that, and the rak’ah will be valid and will count. 
    If he cannot make up what he missed and then catch up with the imam, because 
    that is not possible, then the rak’ah is not valid, rather it is cancelled 
    out because an essential part of it has been omitted. End quote.