Narrated Abu Sa`id Al-Mu'alla: While I was praying, the Prophet (ﷺ) called me but I did not respond to his call. Later I said, "O Allah's Apostle! I was praying." He said, "Didn't Allah say: 'O you who believe! Give your response to Allah (by obeying Him) and to His Apostle when he calls you'?" (8.24) He then said, "Shall I not teach you the most superior Surah in the Qur'an?" He said, '(It is), 'Praise be to Allah, the Lord of the worlds. ' (i.e., Surat Al-Fatiha) which consists of seven repeatedly recited Verses and the Magnificent Qur'an which was given to me."
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا عَلِيُّ بْنُ عَبْدِ اللَّهِ، حَدَّثَنَا يَحْيَى بْنُ سَعِيدٍ، حَدَّثَنَا شُعْبَةُ، قَالَ حَدَّثَنِي خُبَيْبُ بْنُ عَبْدِ الرَّحْمَنِ، عَنْ حَفْصِ بْنِ عَاصِمٍ، عَنْ أَبِي سَعِيدِ بْنِ الْمُعَلَّى، قَالَ كُنْتُ أُصَلِّي فَدَعَانِي النَّبِيُّ صلى الله عليه وسلم فَلَمْ أُجِبْهُ قُلْتُ يَا رَسُولَ اللَّهِ إِنِّي كُنْتُ أُصَلِّي. قَالَ " أَلَمْ يَقُلِ اللَّهُ {اسْتَجِيبُوا لِلَّهِ وَلِلرَّسُولِ إِذَا دَعَاكُمْ} ثُمَّ قَالَ أَلاَ أُعَلِّمُكَ أَعْظَمَ سُورَةٍ فِي الْقُرْآنِ قَبْلَ أَنْ تَخْرُجَ مِنَ الْمَسْجِدِ ". فَأَخَذَ بِيَدِي فَلَمَّا أَرَدْنَا أَنْ نَخْرُجَ قُلْتُ يَا رَسُولَ اللَّهِ إِنَّكَ قُلْتَ لأُعَلِّمَنَّكَ أَعْظَمَ سُورَةٍ مِنَ الْقُرْآنِ. قَالَ "{الْحَمْدُ لِلَّهِ رَبِّ الْعَالَمِينَ} هِيَ السَّبْعُ الْمَثَانِي وَالْقُرْآنُ الْعَظِيمُ الَّذِي أُوتِيتُهُ ".
sunnah.com