Imaam does
  rukoo’ when a person has not finished al-Faatihah
Question

                what should the mu'moon do if he is praying
      in congregation and the imaam goes into rukoo' and the former has not finished reciting
      al-faatiha?
Praise be to Allah.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.