Musnad Ahmad 30:3068

Waki’ narrated to us, he said: Shu’bah narrated to us, from al-‘Ala’ ibn ‘Abd al-Rahman, from his father, from Abu Hurayrah, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "Every prayer in which the Opening of the Book (Fatihat al-Kitab) is not recited is deficient, it is deficient, it is deficient, not complete."