Joining Congregational Salah In Ruku
It is matter of Ijtihad, not direct ruling is received from the prophet. The Muslims adopted different ways for this situation. Those who believe to follow the imam, and join him in ruku after a symbolic stay in Qiyam for a while and consider that they have got the rakat. others recite fatiha and then join the imam. some other join the imam and then repeat the rakat after the congregational prayer is done. you can choose any of these forms according to your understanding.