Sahih al-Bukhari 4474
Sahih al-Bukhari · Book 65: Prophetic Commentary on the Qur'an (Tafseer of the Prophet (pbuh)) · What has been said about Fãtihat al-Kitab (i.e., the Opening of the Book)
Narrated Abu Sa`id bin Al-Mu'alla:
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 " .
While I was praying in the Mosque, Allah's Messenger (ﷺ) called me but I did not respond to him. Later I said, "O Allah's Messenger (ﷺ)! 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."