Musnad Ahmad 30:173

Sufyan narrated to us, Al-Ala’ ibn Abd al-Rahman ibn Ya’qub al-Huraqi informed me—in his house on his bed—from his father, from Abu Hurairah: "Any prayer in which the Opening of the Book (Fatihat al-Kitab) is not recited is deficient (khidaj), then it is deficient, then it is deficient." He said: Abu Hurairah said: And before that, my beloved—upon him be prayer and peace—said: "O Persian, recite the Opening of the Book, for I heard the Messenger of Allah (peace and blessings of Allah be upon him) say: 'Allah the Almighty said: I have divided the prayer between Myself and My servant—and he said once: And for My servant is what he asked. So when he says: {All praise is due to Allah, Lord of the worlds}, He says: My servant has praised Me. When he says: {The Entirely Merciful, the Especially Merciful}, He says: My servant has glorified Me, or My servant has extolled Me. When he says: {Master of the Day of Judgment}, He says: My servant has entrusted [the matter] to Me. When he says: {It is You we worship and You we ask for help}, He says: This is between Me and My servant, and for My servant is what he asked—and he said once: what he asked Me. Then his servant asks Him: {Guide us to the straight path, the path of those upon whom You have bestowed favor, not of those who have evoked [Your] anger or of those who are astray}. He says: This is for My servant, and for you is what you asked—and he said once: And for My servant is what he asked Me.'"