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)   

كُنْتُ أُصَلِّي فَمَرَّ بِي رَسُولُ اللهِ ﷺ فَدَعَانِي فَلَمْ آتِهِ حَتَّى صَلَّيْتُ ثُمَّ أَتَيْتُهُ فَقَالَ مَامَنَعَكَ أَنْ تَأْتِيَنِي؟ فَقَالَ إِنِّي كُنْتُ أُصَلِّي قَالَ أَلَمْ يَقُلِ اللهُ تَبَارَكَ وَتَعَالَى {يَا أَيُّهَا الَّذِينَ آمَنُوا اسْتَجِيبُوا لِلَّهِ وَلِلرَّسُولِ إِذَا دَعَاكُمْ لِمَا يُحْيِيكُمْ} [الأنفال 24] ثُمَّ قَالَ أَلَا أُعَلِّمُكُمْ أَعْظَمَ سُورَةٍ فِي الْقُرْآنِ قَبْلَ أَنْ أَخْرُجَ مِنَ الْمَسْجِدِ قَالَ فَذَهَبَ رَسُولُ اللهِ ﷺ لِيَخْرُجَ فَذَكَّرْتُهُ فَقَالَ الْحَمْدُ لِلَّهِ رَبِّ الْعَالَمِينَ هِيَ السَّبْعُ الْمَثَانِي وَالْقُرْآنُ الْعَظِيمُ الَّذِي أُوتِيتُهُ