Abu Dawud Book 2, Hadith Number 1453.

Chapter : On reciting surah alfatihah.

Narated By Abu Sa’id al-Mu’alla : When I was praying, the Prophet (PBUH) passed me and called me. He said: I prayed and then came to him. He said: What prevented you from answering me? He replied: I was praying. He said: Has not Allah said: “O you who beleive, respond to Allah and the Apostle when he calls you to which gives you life? Let me teach you the greatest surah from the Qur’an or in the Qur’an (the narrator Khalid doubted) before I leave the mosque. I said: (I shall memorise) your saying. He said: It is: “Praise be to Allah, the Lord of the Universe,” which is seven oft repeated verses, and the mighty Qur’an.