Abu Sa'id Ar-Rafi' bin Al-Mu'alla (May Allah be pleased with him) reported:
The Messenger of Allah ﷺ said, "Shall I teach you the greatest Surah in the Qur'an before you leave the mosque?" Then he ﷺ, took me by the hand, and when we were about to step out, I reminded him of his promise to teach me the greatest Surah in the Qur'an. He ﷺ said, "It is 'Alhamdu lillahi Rabbil 'Alamin (i.e., Surat Al-Fatihah) which is As-Sab' Al-Mathani (i.e., the seven oft- repeated Ayat) and the Great Qur'an which is given to me."

[Al-Bukhari].

عن أبي سعيد رافع بن المعلى رضي الله عنه قال : قال لي رسول الله ﷺ :"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 في القرآن ؟ قال: "الحمد لله رب العالمين هي السبع المثاني، والقرآن العظيم الذي أوتيته ". ((رواه البخاري)).