Musnad Ahmad 30:3189

Isma'il narrated to us, from Ibn Jurayj, who said: Al-'Ala' ibn 'Abd al-Rahman ibn Ya'qub informed me that Abu al-Sa'ib informed him that he heard Abu Hurayrah say: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever performs a prayer in which he does not recite the Mother of the Qur'an (Al-Fatiha), it is deficient, it is deficient, it is deficient, not complete." I said: "O Abu Hurayrah, sometimes I am behind the Imam." He squeezed my forearm and said: "O Persian, recite it to yourself."