Narrated Abu Said bin Al-Mu'alla:
While I was praying, Allah's Apostle ﷺ passed me and called me, but I did not go to him until I had finished the prayer. Then I went to him, and he said, "What prevented you from coming to me? Didn't Allah say:-- "O you who believe! Answer the call of Allah (by obeying Him) and His Apostle when He calls you?" He then said, "I will inform you of the greatest Sura in the Qur'an before I leave (the mosque)." When Allah's Apostle got ready to leave (the mosque), I reminded him. He said, "It is: 'Praise be to Allah, the Lord of the worlds.' (i.e. Surat-al-Fatiha) As-sab'a Al-Mathani (the seven repeatedly recited Verses)."
Sahih al-Bukhari, Vol. 6, Book of Prophetic Commentary on the Qur'an, Hadith 170
Sahih al-Bukhari, Book of Prophetic Commentary on the Qur'an, Hadith 170