Abu Hurairah reported the Messenger of Allah (ﷺ) as saying:
If anyone observes a prayer in which he does not recite Umm al-Qur'an, it is incomplete, it is incomplete, it is incomplete, and deficient. (The narrator said) I said: Abu Hurairah, sometime I pray behind the imam(then what should I do)? Pressing my hand he replied: O Persian, recite it inwardly, for I heard the Messenger of Allah (ﷺ) as saying that Allah, Most High, has said: I have Me and the Half for my servant and My servant will receive what he asks. The Messenger of Allah (ﷺ) said: Recite. When the servant says: "praise be to Allah, the Lord of the Universe," Allah, Most High says: "My servant has praised me." When the servant says: " The Compassionate, the merciful, "Allah Most High says: "My servant has lauded me." When the servant says: "Owner of the Day of Judgment," Allah, Most High, says: "My servant has glorified Me" When the servant says: " You do we worship and of you we ask help. " (Allah says) "This is between Me and My servant, and My servant will receive what he asks." When the servant says: " Guide us to the Straight Path, the path of those whom you have favoured, not ( the path) of those who earn thine anger nor of those who go astray," (Allah says: ) "This is for My servant, and My servant will receive what he asks."