The Messenger of Allah ﷺ said to me, "Shall I not teach you the greatest surah in the Quran before I leave the mosque?"Then the Prophet ﷺ went to leave, so I reminded him. He said, "It is Alhamdu lillahi rabbil-alameen. It is the seven oft-repeated verses and the Great Quran that has been given to me."
حدثنا أبو بكر بن أبي شيبة حدثنا غندر عن شعبة عن خبيب بن عبد الرحمن عن حفص بن عاصم
قال لي رسول الله ﷺ ألا أعلمك أعظم سورة في القرآن قبل أن أخرج من المسجد قال فذهب النبي ﷺ ليخرج فأذكرته فقال الحمد لله رب العالمين وهي السبع المثاني والقرآن العظيم الذي أوتيته
Bukhari narrated it in the Book of Tafsir, chapter of His saying, "And We have certainly given you seven of the often-repeated [verses] and the great Quran," hadith no. 4703, through Ghundar, with the same chain.