The Prophet ﷺ passed by me while I was praying. He called me, but I did not respond until I had finished my prayer. Then I came to him.He asked, "What kept you from coming?"I replied, "I was praying."He said, "Did Allah not say, 'You who believe, respond to Allah and the Messenger when he calls you to that which gives you life. And know that Allah comes between a man and his heart, and that to Him you will be gathered' [Anfal 8:24]?"Then he said, "Shall I teach you the greatest surah in the Quran before I leave the mosque?"The Prophet ﷺ then went to leave the mosque, and I reminded him. He said, "'Praise be to Allah, Lord of the worlds' [Fatihah 1:2]. It is the seven oft-repeated verses and the great Quran that has been given to me."
مر بي النبي ﷺ، وأنا أصلي، فدعاني، فلم أنه حتى صليت، ثم أتيت، فقال: ما منعك أن تأتي؟ فقلت: كنت أصلى. فقال: ألم يقل الله {يأيها الذين ءامنوا استجيبوا لله وللرسول إذا دعاكم لما يحييكم واعلموا أن الله يحول بين المرء وقلبه وأنه إليه تحشرون} [الأنفال: ٢٤]. ثم قال: ألا أعلمك أعظم سورة في القرآن قبل أن أخرج من المسجد. فذهب النبي ﷺ ليخرج من المسجد، فذكرته، فقال: {الحمد لله رب العلمين} هي السبع المثاني، والقرآن العظيم الذي أوتيته.
Bukhari related it in the tafsir (4703), from Muhammad bin Bashshar, who said, Ghundar narrated to us, who said, Shubah narrated to us, from Khubayb bin Abdul-Rahman, from Hafs bin Asim, from Abu Saeed bin Muallah, who mentioned it.