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].