Musnad Abu Ya'la al-Mawsili 160:59

Abu Khaythama narrated to us, Sa'id bin 'Amir narrated to us, from Shu'bah, from al-'Ala' bin 'Abd al-Rahman, from his father, from Abu Hurayrah, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Every prayer in which the Mother of the Qur'an (al-Fatiha) is not recited is deficient, it is deficient, it is deficient, not complete." They said: "O Abu Hurayrah, what if I am behind the Imam?" He said: "Recite it to yourself, O Persian."