He cannot prostrate on the ground when he prays in congregation
Question

                I have a medical problem with my knees, and I can only bend them with difficulty. When I pray in congregation I cannot prostrate on the ground because of the crowding in the row and the fact that I need to move a great deal. So I am forced to prostrate in the air (i.e., by bending forward slightly). But if I pray on my own at home, I am able to prostrate on the ground with all seven parts of the body. So should I pray with the congregation in the mosque or on my own at home?
Praise be to Allah.
    You should pray in the mosque, in congregation, because 
    praying in congregation is obligatory, and you should prostrate in the 
    manner that you are able, even if it is by gesturing. In this case you will 
    not be regarded as having failed to prostrate.

     This answer was given to us by Shaykh ‘Abd al-Rahmaan 
    al-Barraak, and we have paraphrased it.

     And Allaah knows best.