Praise be to Allaah.
The person who is praying behind the imaam should do rukoo as soon as the imaam does rukoo, because the Prophet (peace and blessings of Allaah be upon him) said: The imaam is appointed only to be followed, so when he prays standing, pray standing; when he bows (does rukoo), bow; when he stands up again, stand up again; when he says Sami Allaahu liman hamidah (Allaah hears the one who praises Him), say, Rabbanaa wa lakal-hamd (Our Lord, to You be praise). If he prays standing, then pray standing, and if he prays sitting, then pray sitting, all of you. (Reported by al-Bukhaari in his Saheeh, 648). The person who is praying behind an imaam is excused for not completing al-Faatihah in this case, because he is following the imaam. And Allaah knows best.
Sheikh Muhammed Salih Al-Munajjid