Narrated Abu Sa`id bin Al-Mu'alla: While I was praying, Allah's Messenger (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 Allah's Messenger (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).
Source: sunnah.com
Other translations
Arabic
حَدَّثَنِي إِسْحَاقُ، أَخْبَرَنَا رَوْحٌ، حَدَّثَنَا شُعْبَةُ، عَنْ خُبَيْبِ بْنِ عَبْدِ الرَّحْمَنِ، سَمِعْتُ حَفْصَ بْنَ عَاصِمٍ، يُحَدِّثُ عَنْ أَبِي سَعِيدِ بْنِ الْمُعَلَّى ـ رضى الله عنه ـ قَالَ كُنْتُ أُصَلِّي فَمَرَّ بِي رَسُولُ اللَّهِ صلى الله عليه وسلم 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 صلى الله عليه وسلم لِيَخْرُجَ فَذَكَرْتُ لَهُ. وَقَالَ مُعَاذٌ حَدَّثَنَا شُعْبَةُ، عَنْ خُبَيْبٍ، سَمِعَ حَفْصًا، سَمِعَ أَبَا سَعِيدٍ، رَجُلاً مِنْ أَصْحَابِ النَّبِيِّ صلى الله عليه وسلم بِهَذَا، وَقَالَ هِيَ {الْحَمْدُ لِلَّهِ رَبِّ الْعَالَمِينَ} السَّبْعُ الْمَثَانِي.
sunnah.com