Sahih al-Bukhari, Volume 6, Book 60, Hadith 1 (USC-MSA) ⓘ

Narrated Abu Sa`id bin Al-Mu'alla: While I was praying in the Mosque, Allah's Messenger called me but I did not respond to him. Later I said, "O Allah's Messenger! I was praying." He said, "Didn't Allah say'--"Give your response to Allah (by obeying Him) and to His Apostle when he calls you." (8.24) He then said to me, "I will teach you a Sura which is the greatest Sura in the Qur'an, before you leave the Mosque." Then he got hold of my hand, and when he intended to leave (the Mosque), I said to him, "Didn't you say to me, 'I will teach you a Sura which is the greatest Sura in the Qur'an?' He said, "Al-Hamdu-Li l-lah Rabbi-l-`alamin (i.e. Praise be to Allah, the Lord of the worlds) which is Al-Sab'a Al-Mathani (i.e. seven repeatedly recited Verses) and the Grand Qur'an which has been given to me."

حَدَّثَنَا مُسَدَّدٌ، حَدَّثَنَا يَحْيَى، عَنْ شُعْبَةَ، قَالَ حَدَّثَنِي خُبَيْبُ بْنُ عَبْدِ الرَّحْمَنِ، عَنْ حَفْصِ بْنِ عَاصِمٍ، عَنْ أَبِي سَعِيدِ بْنِ الْمُعَلَّى، قَالَ كُنْتُ أُصَلِّي فِي الْمَسْجِدِ فَدَعَانِي رَسُولُ اللَّهِ صلى الله عليه وسلم فَلَمْ أُجِبْهُ، فَقُلْتُ يَا رَسُولَ اللَّهِ إِنِّي كُنْتُ أُصَلِّي‏.‏ فَقَالَ ‏"‏ أَلَمْ يَقُلِ اللَّهُ ‏{‏اسْتَجِيبُوا لِلَّهِ وَلِلرَّسُولِ إِذَا دَعَاكُمْ‏}‏ ثُمَّ قَالَ لِي لأُعَلِّمَنَّكَ سُورَةً هِيَ أَعْظَمُ السُّوَرِ فِي الْقُرْآنِ قَبْلَ أَنْ تَخْرُجَ مِنَ الْمَسْجِدِ ‏"‏‏.‏ ثُمَّ أَخَذَ بِيَدِي، فَلَمَّا أَرَادَ أَنْ يَخْرُجَ قُلْتُ لَهُ أَلَمْ تَقُلْ ‏"‏ لأُعَلِّمَنَّكَ سُورَةً هِيَ أَعْظَمُ سُورَةٍ فِي الْقُرْآنِ ‏"‏‏.‏ قَالَ ‏"‏ ‏{‏الْحَمْدُ لِلَّهِ رَبِّ الْعَالَمِينَ‏}‏ هِيَ السَّبْعُ الْمَثَانِي وَالْقُرْآنُ الْعَظِيمُ الَّذِي أُوتِيتُهُ ‏"‏‏.‏