It is narrated from Hazrat Abu Hurairah (may Allah be pleased with him) that the Noble Prophet (peace and blessings be upon him) said: Any prayer in which Surah Al-Fatihah is not recited is incomplete, incomplete, incomplete. I asked, "O Abu Hurairah! What if sometimes I am behind the Imam?" He pinched my arm and said, "O Persian, recite it in your heart."
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 10319