Abu Sa'id b. al-Mu'alla said that when he was praying the Prophet ﷺ passed by him and he called him. He said:

I prayed ant then I came to him. He asked: What prevented you from answering me ? He replied: I was praying. He said: Has not Allah said: "O you who believe, respond to Allah and the Apostle when he calls you to that which gives you life ? (8:24) Let me teach you the greatest surah from the Qur'an or in the Qur'an (the narrator Khalid doubted) before I leave the mosque. I said: (I shall memorize) your saying. He said: It is: "Praise be to Allah, the Lord of the Universe" which is the seven oft-repeated verses, and the mighty Qur'an.

حَدَّثَنَا عُبَيْدُ اللَّهِ بْنُ مُعَاذٍ، حَدَّثَنَا خَالِدٌ، حَدَّثَنَا شُعْبَةُ، عَنْ خُبَيْبِ بْنِ عَبْدِ الرَّحْمَنِ، قَالَ سَمِعْتُ حَفْصَ بْنَ عَاصِمٍ، يُحَدِّثُ عَنْ أَبِي سَعِيدِ بْنِ الْمُعَلَّى، أَنَّ النَّبِيَّ ﷺ مَرَّ بِهِ وَهُوَ يُصَلِّي فَدَعَاهُ قَالَ فَصَلَّيْتُ ثُمَّ أَتَيْتُهُ قَالَ فَقَالَ " مَا مَنَعَكَ أَنْ تُجِيبَنِي " . قَالَ كُنْتُ أُصَلِّي . قَالَ " أَلَمْ يَقُلِ اللَّهُ عَزَّ وَجَلَّ { يَا أَيُّهَا الَّذِينَ آمَنُوا اسْتَجِيبُوا لِلَّهِ وَلِلرَّسُولِ إِذَا دَعَاكُمْ لِمَا يُحْيِيكُمْ } لأُعَلِّمَنَّكَ أَعْظَمَ سُورَةٍ مِنَ الْقُرْآنِ أَوْ فِي الْقُرْآنِ " . شَكَّ خَالِدٌ " قَبْلَ أَنْ أَخْرُجَ مِنَ الْمَسْجِدِ " . قَالَ قُلْتُ يَا رَسُولَ اللَّهِ قَوْلَكَ . قَالَ " { الْحَمْدُ لِلَّهِ رَبِّ الْعَالَمِينَ } وَهِيَ السَّبْعُ الْمَثَانِي الَّتِي أُوتِيتُ وَالْقُرْآنُ الْعَظِيمُ " .