Mishkat al-Masabih 2118
Abū Sa'īd b. al-Mu‘allā said:
When I was praying in the mosque the Prophet called me and I did not answer him, but I went to him afterwards and explained that I had been praying, whereupon he asked me whether God had not said, “Respond to God and to the messenger when He calls you,” (Qur’ān, 8:24)  adding, “Let me teach you the greatest sūra in the Qur’ān before you leave the mosque.” He then took me by the hand, and when we were about to go out I reminded him of saying he would teach me the greatest sūra in the Qur’ān. He said, “It is, ‘Praise be to God, the Lord of the universe’, (Qur'ān,1) which is the seven oft-repeated verses and the mighty Qur’ān’ (Qur’ān, 15:87) which has been brought to me.”  

Bukhārī transmitted it.
وَعَنْ أَبِي سَعِيدٍ بْنِ الْمُعَلَّى قَالَ: كُنْتُ أُصَلِّي فِي الْمَسْجِدِ فَدَعَانِي النَّبِيُّ صَلَّى اللَّهُ عَلَيْهِ وَسلم فَلم أجبه حَتَّى صليت ثُمَّ أَتَيْتُهُ. فَقُلْتُ يَا رَسُولَ اللَّهِ إِنِّي كنت أُصَلِّي فَقَالَ أَلَمْ يَقُلِ اللَّهُ (اسْتَجِيبُوا لِلَّهِ وَلِلرَّسُولِ إِذَا دعَاكُمْ)
ثمَّ قَالَ لي: «أَلَا أُعَلِّمُكَ أَعْظَمَ سُورَةٍ فِي الْقُرْآنِ قَبْلَ أَنْ تَخْرُجَ مِنَ الْمَسْجِدِ» . فَأَخَذَ بِيَدِي فَلَمَّا أَرَادَ أَن يخرج قلت لَهُ ألم تقل لأعلمنك سُورَة هِيَ أعظم سُورَةً مِنَ الْقُرْآنِ قَالَ: (الْحَمْدُ لِلَّهِ رَبِّ الْعَالَمِينَ)
هِيَ السَّبْعُ الْمَثَانِي وَالْقُرْآنُ الْعَظِيمُ الَّذِي أُوتِيتهُ ". رَوَاهُ البُخَارِيّ