Which is better, to join the imam whilst he is prostrating or to wait until he gets up?
Question

                Is it permissible for the worshipper who missed a rak’ah and found the imam prostrating to say takbeer and then prostrate, or should he wait until the imam gets up?.
Praise be to Allah.
    If a person comes and finds the imam in a particular posture 
    he should join the imam in whatever posture he is in. So if he comes and 
    finds him prostrating, he should say takbeer and then prostrate, because the 
    Prophet  (peace and blessings of Allaah be upon him) said: “Whatever you 
    catch up with, pray, and whatever you missed, make it up.” And because the 
    prostration that he does may be a cause of forgiveness of his sins, so he 
    should not miss out on that. Hence he should prostrate and then stand up 
    with his imam, but that is not counted as a rak’ah for him, because the 
    rak’ah can only be caught up with if one catches up with the bowing 
    (rukoo’). End quote. 

    Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him).