Musannaf Ibn Abi Shayba 3:1242

Isma'il ibn 'Ulayyah narrated to us, from Ibn Jurayj, from al-'Ala' ibn 'Abd al-Rahman ibn Ya'qub, that Abu al-Sa'ib informed him that he heard Abu Hurayrah saying: The Messenger of Allah (ﷺ) said: "Whoever performs a prayer in which he does not recite the Opening of the Book (Fatihat al-Kitab), it is deficient, it is deficient, it is deficient, not complete."