"I heard Abu Hurairah say: 'The Messenger of Allah ﷺ said: "Whoever offers a prayer in which he does not recite Umm Al-Quran (Al Fatihah), it is deficient, it is deficient, it is deficient, incomplete." I (Abu As-Sa'ib) said: 'O Abu Hurairah, sometimes I am behind the Imam.' He poked me in the arm and said: 'Recite it to yourself, O Persian! For I heard the Messenger of Allah ﷺ say: "Allah says: "I have divided prayer between Myself and My slave into two halves, and My slave shall have what he has asked for.'" The Messenger of Allah ﷺ said: "Recite, for when the slave says: All the praises and thanks be to Allah, the Lord of all that exists, Allah says: 'My slave has praised Me.' And when he says: The Most Gracious, the Most Merciful, Allah says: 'My slave has extolled Me.' And when he says: The Only Owner (and the Only Ruling Judge) of the Day of Recompense (i.e. the Day of Resurrection), Allah says: 'My slave has glorified Me' . And when he says: You (alone) we worship, and You (alone) we ask for help (for each and everything), He says: 'This is between Me and My slave, and My slave shall have what he has asked for.' And when he says: 'Guide us to the straight way, the way of those on whom You have bestowed Your grace, not (the way) of those who earned Your anger, nor of those who went astray, He says: 'This is for My slave, and My slave shall have what he asked for.'" (Using translation from Nasāʾī 909)  

قَالَ رَسُولُ اللهِ ﷺ «مَنْ صَلَّى صَلَاةً لَمْ يَقْرَأْ فِيهَا بِأُمِّ الْقُرْآنِ فَهِيَ خِدَاجٌ هِيَ خِدَاجٌ هِيَ خِدَاجٌ غَيْرُ تَمَامٍ» فَقُلْتُ يَا أَبَا هُرَيْرَةَ إِنِّي أَحْيَانًا أَكُونُ وَرَاءَ الْإِمَامِ فَغَمَزَ ذِرَاعِي وَقَالَ اقْرَأْ بِهَا يَا فَارِسِيُّ فِي نَفْسِكَ فَإِنِّي سَمِعْتُ رَسُولَ اللهِ ﷺ يَقُولُ قَالَ اللهُ قَسَمْتُ الصَّلَاةَ بَيْنِي وَبَيْنَ عَبْدِي نِصْفَيْنِ فَنِصْفُهَا لِي وَنِصْفُهَا لِعَبْدِي وَلِعَبْدِي مَا سَأَلَ قَالَ رَسُولُ اللهِ ﷺ اقْرَءُوا يَقُولُ الْعَبْدُ {الْحَمْدُ لِلَّهِ رَبِّ الْعَالَمِينَ} [الفاتحة 2] يَقُولُ اللهُ حَمِدَنِي عَبْدِي يَقُولُ الْعَبْدُ {الرَّحْمَنِ الرَّحِيمِ} [الفاتحة 1] يَقُولُ اللهُ أَثْنَى عَلَيَّ عَبْدِي يَقُولُ الْعَبْدُ {مَالِكِ يَوْمِ الدِّينِ} يَقُولُ اللهُ مَجَّدَنِي عَبْدِي يَقُولُ الْعَبْدُ {إِيَّاكَ نَعْبُدُ وَإِيَّاكَ نَسْتَعِينُ} [الفاتحة 5] فَهَذِهِ الْآيَةُ بَيْنِي وَبَيْنَ عَبْدِي وَلِعَبْدِي مَا سَأَلَ يَقُولُ الْعَبْدُ {اهْدِنَا الصِّرَاطَ الْمُسْتَقِيمَ صِرَاطَ الَّذِينَ أَنْعَمْتَ عَلَيْهِمْ غَيْرِ الْمَغْضُوبِ عَلَيْهِمْ وَلَا الضَّالِّينَ} [الفاتحة 7] فَهَؤُلَاءِ لِعَبْدِي وَلِعَبْدِي مَا سَأَلَ