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 دَعَاهُ وَهُوَ يُصَلِّي فَصَلَّى ثُمَّ أَتَاهُ فَقَالَ مَا مَنَعَكَ أَنْ تُجِيبَنِي إِذْ دَعَوْتُكَ؟ قَالَ إِنِّي كُنْتُ أُصَلِّي فَقَالَ أَلَمْ يَقُلِ اللهُ ﷻ {يَا أَيُّهَا الَّذِينَ آمَنُوا اسْتَجِيبُوا لِلَّهِ وَلِلرَّسُولِ إِذَا دَعَاكُمْ} [الأنفال 24] الْآيَةَ ثُمَّ قَالَ أَلَا أُعَلِّمُكَ أَعْظَمَ سُورَةٍ فِي الْقُرْآنِ؟ قَالَ فَكَأَنَّهُ نَسِيَهَا أَوْ نَسِيَ قُلْتُ يَا رَسُولَ اللهِ الَّذِي قُلْتَ لِي قَالَ الْحَمْدُ لِلَّهِ رَبِّ الْعَالَمِينَ هِيَ السَّبْعُ الْمَثَانِي وَالْقُرْآنُ الْعَظِيمُ الَّذِي أُوتِيتُهُ