Abu Sa`id Ar-Rafi` bin Al-Mu`alla (May Allah be pleased with him) reported: The Messenger of Allah (PBUH) said, "Shall I teach you the greatest Surah in the Qur'an before you leave the mosque?'' Then he (PBUH), 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 (PBUH) 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].