Sunan Abi Dawud 821
Abu Hurairah reported the Messenger of Allah (May peace be upon him) 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: “ Thee do we worship and of thee 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 thou hast 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.”
حَدَّثَنَا الْقَعْنَبِيُّ، عَنْ مَالِكٍ، عَنِ الْعَلاَءِ بْنِ عَبْدِ الرَّحْمَنِ، أَنَّهُ سَمِعَ أَبَا السَّائِبِ، مَوْلَى هِشَامِ بْنِ زُهْرَةَ يَقُولُ سَمِعْتُ أَبَا هُرَيْرَةَ، يَقُولُ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏"‏ مَنْ صَلَّى صَلاَةً لَمْ يَقْرَأْ فِيهَا بِأُمِّ الْقُرْآنِ فَهِيَ خِدَاجٌ فَهِيَ خِدَاجٌ فَهِيَ خِدَاجٌ غَيْرُ تَمَامٍ ‏"‏ ‏.‏ قَالَ فَقُلْتُ يَا أَبَا هُرَيْرَةَ إِنِّي أَكُونُ أَحْيَانًا وَرَاءَ الإِمَامِ ‏.‏ قَالَ فَغَمَزَ ذِرَاعِي وَقَالَ اقْرَأْ بِهَا يَا فَارِسِيُّ فِي نَفْسِكَ فَإِنِّي سَمِعْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم يَقُولُ ‏"‏ قَالَ اللَّهُ تَعَالَى قَسَمْتُ الصَّلاَةَ بَيْنِي وَبَيْنَ عَبْدِي نِصْفَيْنِ فَنِصْفُهَا لِي وَنِصْفُهَا لِعَبْدِي وَلِعَبْدِي مَا سَأَلَ ‏"‏ ‏.‏ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏"‏ اقْرَءُوا يَقُولُ الْعَبْدُ ‏{‏ الْحَمْدُ لِلَّهِ رَبِّ الْعَالَمِينَ ‏}‏ يَقُولُ اللَّهُ عَزَّ وَجَلَّ حَمِدَنِي عَبْدِي يَقُولُ الْعَبْدُ ‏{‏ الرَّحْمَنِ الرَّحِيمِ ‏}‏ يَقُولُ اللَّهُ عَزَّ وَجَلَّ أَثْنَى عَلَىَّ عَبْدِي يَقُولُ الْعَبْدُ ‏{‏ مَالِكِ يَوْمِ الدِّينِ ‏}‏ يَقُولُ اللَّهُ عَزَّ وَجَلَّ مَجَّدَنِي عَبْدِي يَقُولُ الْعَبْدُ ‏{‏ إِيَّاكَ نَعْبُدُ وَإِيَّاكَ نَسْتَعِينُ ‏}‏ يَقُولُ اللَّهُ وَهَذِهِ بَيْنِي وَبَيْنَ عَبْدِي وَلِعَبْدِي مَا سَأَلَ يَقُولُ الْعَبْدُ ‏{‏ اهْدِنَا الصِّرَاطَ الْمُسْتَقِيمَ * صِرَاطَ الَّذِينَ أَنْعَمْتَ عَلَيْهِمْ غَيْرِ الْمَغْضُوبِ عَلَيْهِمْ وَلاَ الضَّالِّينَ ‏}‏ يَقُولُ اللَّهُ فَهَؤُلاَءِ لِعَبْدِي وَلِعَبْدِي مَا سَأَلَ ‏"‏ ‏.‏