The Prophet ﷺ passed by him when he was praying, and called him. He said: "I finished praying, then I came to him, and he said: 'What kept you from answering me?' He said: 'I was praying.' He said: 'Does not Allah say: O you who believe! Answer Allah (by obeying Him) and (His) Messenger when he calls you to that which will give you life? Shall I not teach you the greatest surah before I leave the masjid?' Then he went to leave, and I said: 'O Messenger of Allah, what about what you said?' He said: "All praise and thanks be to Allah, Lord of all that exists. These are the seven oft-recited that I have been given, and the Grand Quran.'" (Using translation from Nasāʾī 913)  

أَنَّ النَّبِيَّ ﷺ مَرَّ بِهِ وَهُوَ يُصَلِّي فَدَعَاهُ قَالَ فَصَلَّيْتُ ثُمَّ أَتَيْتُهُ قَالَ «مَا مَنَعَكَ أَنْ تُجِيبَنِي؟» قَالَ كُنْتُ أُصَلِّي قَالَ أَلَمْ يَقُلِ اللهُ ﷻ {يَا أَيُّهَا الَّذِينَ آمَنُوا اسْتَجِيبُوا لِلَّهِ وَلِلرَّسُولِ إِذَا دَعَاكُمْ لِمَا يُحْيِيكُمْ} [الأنفال 24] أَلَا أُعَلِّمُكَ أَعْظَمَ سُورَةٍ قَبْلَ أَنْ أَخْرُجَ مِنَ الْمَسْجِدِ؟ قَالَ فَذَهَبَ لِيُخْرِجَ قُلْتُ يَا رَسُولَ اللهِ قَوْلَكَ قَالَ الْحَمْدُ لِلَّهِ رَبِّ الْعَالَمِينَ هِيَ السَّبْعُ الْمَثَانِيَ وَالْقُرْآنُ الْعَظِيمُ  

"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 «كُلُّ صَلَاةٍ لَمْ يُقْرَأْ فِيهَا بِأُمِّ الْقُرْآنِ هِيَ خِدَاجٌ هِيَ خِدَاجٌ هِيَ خِدَاجٌ غَيْرُ تَمَامٍ» قُلْتُ يَا أَبَا هُرَيْرَةَ إِنِّي أَكُونُ أَحْيَانًا وَرَاءَ الْإِمَامِ؟ فَغَمَزَ ذِرَاعِي وَقَالَ اقْرَأْ بِهَا يَا فَارِسِيُّ فِي نَفْسِكَ فَإِنِّي سَمِعْتُ رَسُولَ اللهِ ﷺ يَقُولُ قَالَ اللهُ ﷻ 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 تَبَارَكَ وَتَعَالَى 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  

Abu Hurairah reported the prophet ﷺ as saying; when the imam recites “ not of those with whom thou art angry, nor of those who go astray” (surah al-fatihah,verse 7) say Amin, for if one’s words (utterance of amin) synchronise with those of the angles, he will be forgiven his past sins. (Using translation from Abū Dāʾūd 935)  

أَنَّ رَسُولَ اللهِ ﷺ قَالَ إِذَا قَالَ الْإِمَامُ {غَيْرِ الْمَغْضُوبِ عَلَيْهِمْ وَلَا الضَّالِّينَ} [الفاتحة 7] فَقُولُوا آمِينَ فَإِنَّهُ مَنْ وَافَقَ قَوْلُهُ قَوْلَ الْمَلَائِكَةِ غُفِرَ لَهُ مَا تَقَدَّمَ مِنْ ذَنْبِهِ