Narrated Abu Sa`id bin Al-Mu'alla: While I was praying, Allah's Messenger 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 Messenger 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).