Musnad Ahmad 30:288

Isma'il narrated to us, from Ibn Jurayj, that al-'Ala' ibn 'Abd al-Rahman ibn Ya'qub informed him 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 Book (Umm al-Kitab), it is deficient, it is deficient, it is deficient, not complete." I said: "O Abu Hurayrah, I am sometimes behind the Imam." He pinched my arm and said: "O Persian, recite it to yourself."