Sahih al-Bukhari 4474
Prophetic Commentary on the Qur'an (Tafseer of the Prophet (pbuh))
What has been said about Fãtihat al-Kitab (i.e., the Opening of the Book)
Narrated Abu Sa`id bin Al-Mu'alla: While I was praying in the Mosque, Allah's Messenger (ﷺ) called me but I did not respond to him. Later I said, "O Allah's Messenger (ﷺ) ! I was praying." He said, "Didn't Allah say'--"Give your response to Allah (by obeying Him) and to His Messenger when he calls you." (8:24) He then said to me, "I will teach you a Sura which is the greatest Sura in the Qur'an, before you leave the Mosque." Then he got hold of my hand, and when he intended to leave (the Mosque), I said to him, "Didn't you say to me, 'I will teach you a Sura which is the greatest Sura in the Qur'an?' He said, "Al-Hamdu-Li l-lah Rabbi-l-`alamin (i.e. Praise be to Allah, the Lord of the worlds) which is Al-Sab'a Al-Mathani (i.e. seven repeatedly recited Verses) and the Grand Qur'an which has been given to me."