While I was praying, Messenger of Allah ﷺ passed me and called me, but I did not go to him until I had finished the prayer. Then I went to him, and he said, "What prevented you from coming to me? Didn't Allah say:-- "O you who believe! Answer the call of Allah (by obeying Him) and His Apostle when He calls you?" He then said, "I will inform you of the greatest Sura in the Qur'an before I leave (the mosque)." When Messenger of Allah ﷺ got ready to leave (the mosque), I reminded him. He said, "It is: 'Praise be to Allah, the Lord of the worlds.' (i.e. Surat-al-Fatiha) As-sab'a Al-Mathani (the seven repeatedly recited Verses).
كُنْتُ أُصَلِّي فَمَرَّ بِي رَسُولُ اللَّهِ ﷺ فَدَعَانِي فَلَمْ آتِهِ حَتَّى صَلَّيْتُ ثُمَّ أَتَيْتُهُ فَقَالَ مَا مَنَعَكَ أَنْ تَأْتِيَ أَلَمْ يَقُلِ اللَّهُ {يَا أَيُّهَا الَّذِينَ آمَنُوا اسْتَجِيبُوا لِلَّهِ وَلِلرَّسُولِ إِذَا دَعَاكُمْ} ثُمَّ قَالَ لأُعَلِّمَنَّكَ أَعْظَمَ سُورَةٍ فِي الْقُرْآنِ قَبْلَ أَنْ أَخْرُجَ فَذَهَبَ رَسُولُ اللَّهِ ﷺ لِيَخْرُجَ فَذَكَرْتُ لَهُ وَقَالَ مُعَاذٌ حَدَّثَنَا شُعْبَةُ عَنْ خُبَيْبٍ سَمِعَ حَفْصًا سَمِعَ أَبَا سَعِيدٍ رَجُلاً مِنْ أَصْحَابِ النَّبِيِّ ﷺ بِهَذَا وَقَالَ هِيَ {الْحَمْدُ لِلَّهِ رَبِّ الْعَالَمِينَ} السَّبْعُ الْمَثَانِي