I was praying when the Prophet ﷺ called me, and I did not answer him. Later I said, "Messenger of Allah, I was praying."He said, "Did Allah not say, 'Respond to Allah and the Messenger when he calls you to that which gives you life' [Anfal 8:24]?"Then he said, "Shall I not teach you the greatest surah in the Quran before you leave the masjid?" And he took me by the hand.When we were about to leave, I said, "Messenger of Allah, you said you would teach me the greatest surah in the Quran."He said, "It is 'All praise belongs to Allah, Lord of the worlds' [Fatihah 1:2]. It is the seven oft-repeated verses and the mighty Quran that I have been given."
كنت أصلي، فدعاني النبي ﷺ فلم أجبه، قلت: يا رسول الله إني كنت أصلي، قال: ألم يقل الله {استجيبوا لله والرسول إذا دعاكم لما يحييكم} [سورة الأنفال: ٢٤] ثم قال: ألا أعلمك أعظم سورة في القرآن قبل أن تخرج من المسجد؟ فأخذ بيدي، فلما أردنا أن نخرج قلت: يا رسول الله، إنك قلت: لأعلمنك أعظم سورة من القرآن، قال: {الحمد لله رب العالمين} و هي السبع المثاني والقرآن العظيم الذي أوتيته.
Bukhari narrated it in the Virtues of the Quran (5006) from Ali bin Abdullah, who said, Yahya bin Saeed narrated to us, who said, Shubah narrated to us, who said, Khubayb bin Abdul-Rahman narrated to me, from Hafs bin Asim, from Abu Saeed bin Mualla, and he mentioned it.As for what Malik narrated in the Prayer (40) from Ala bin Abdul-Rahman bin Yaqub, that Abu Saeed, the freedman of Amir bin Kurayz, informed him that the Messenger of Allah called out to Ubayy bin Kaab while he was praying. When he finished his prayer, he caught up with him, and the Messenger of Allah placed his hand on his hand while he intended to leave through the door of the masjid, and said, "I do hope that you will not leave the masjid before you learn a surah, the like of which Allah has not sent down in the Torah, nor in the Injil, nor in the Quran."Ubayy said, so I began to walk slowly, hoping for that. Then I said, "O Messenger of Allah, the surah that you promised me." He said, "How do you recite when you begin the prayer?" He said, so I recited "Praise be to Allah, Lord of the worlds" until I reached its end. The Messenger of Allah then said, "This is that surah, it is the seven oft-repeated verses, and the mighty Quran which I have been given."This chain is disconnected, for Abu Saeed, the freedman of Amir bin Kurayz, is a Tabii from the freedmen of Khuza, and he is not Abu Saeed bin Mualla, the narrator in the previous hadith, who was an Ansari companion. As for the story of Ubayy bin Kaab, it has come from another chain that is sahih, which follows. Hafiz bin Kathir drew attention to this in his tafsir of Surah Fatihah.