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.
