Narrated Abu Said Al-Mualla:

While I was praying, the Prophet ﷺ passed by and called me, but I did not go to him till I had finished my prayer. When I went to him, he said, "What prevented you from coming?" I said, "I was praying." He said, "Didn't Allah say" "O you who believes Give your response to Allah (by obeying Him) and to His Apostle." (8.24) Then he added, "Shall I tell you the most superior Sura in the Qur'an before I go out of the mosque?" When the Prophet intended to go out (of the Mosque), I reminded him and he said, "That is: "Alhamdu-lillahi Rabbil-'Alamin (Surat-al-fatiha)' which is the seven oft repeated verses (Al-Mathani) and the Grand Quran which has been given to me."

حَدَّثَنِي مُحَمَّدُ بْنُ بَشَّارٍ، حَدَّثَنَا غُنْدَرٌ، حَدَّثَنَا شُعْبَةُ، عَنْ خُبَيْبِ بْنِ عَبْدِ الرَّحْمَنِ، عَنْ حَفْصِ بْنِ عَاصِمٍ، عَنْ أَبِي سَعِيدِ بْنِ الْمُعَلَّى، قَالَ مَرَّ بِيَ النَّبِيُّ ﷺ وَأَنَا أُصَلِّي فَدَعَانِي فَلَمْ آتِهِ حَتَّى صَلَّيْتُ ثُمَّ أَتَيْتُ فَقَالَ " مَا مَنَعَكَ أَنْ تَأْتِيَ ". فَقُلْتُ كُنْتُ أُصَلِّي. فَقَالَ " أَلَمْ يَقُلِ اللَّهُ يَا أَيُّهَا الَّذِينَ آمَنُوا اسْتَجِيبُوا لِلَّهِ وَلِلرَّسُولِ ثُمَّ قَالَ أَلاَ أُعَلِّمُكَ أَعْظَمَ سُورَةٍ فِي الْقُرْآنِ قَبْلَ أَنْ أَخْرُجَ مِنَ الْمَسْجِدِ " فَذَهَبَ النَّبِيُّ ﷺ لِيَخْرُجَ مِنَ الْمَسْجِدِ فَذَكَّرْتُهُ فَقَالَ "الْحَمْدُ لِلَّهِ رَبِّ الْعَالَمِينَ هِيَ السَّبْعُ الْمَثَانِي وَالْقُرْآنُ الْعَظِيمُ الَّذِي أُوتِيتُهُ ".