If a latecomer
    catches up with the imaam during rukoo’ (bowing)
Question

                If a person catches up with the imaam
      during rukoo’, will the rak’ah count for him?
Praise be to Allah.If the latecomer catches up with the imaam in
    rukoo’, our companions said:
If the person can say ‘Allaahu akbar’ and bow with a pause
    before he stands up again, then the rak’ah counts for him, but if there is no time to
    pause before the imaam stands up again, it does not count. If he is not sure, does the
    rak’ah count? There are two opinions on this, the more correct of which is:
It does not count, because what is more likely is that he did not catch
    up with the imaam. So he has to do the prostration of forgetfulness (sajdat al-sahw)
    at the end of the rak’ah that he does after the imaam says the salaam, because he is
    doing a rak’ah on his own and is not sure whether this is an extra rak’ah or
    not. This is like the one who is not sure whether he has prayed three or four
    rak’ahs: he should do another rak’ah then do sajdat al-sahw. One of those
    who discussed this issue was al-Ghazaali in al-Fatawa. It is a good issue to
    discuss, because a lot of people have this problem and most people do not know the answer,
    so this information should be passed on. And Allaah knows best.