Musnad Ahmad 558:1

Yahya ibn Sa'id narrated to us, from Shu'bah, who said: Khubayb ibn 'Abd al-Rahman narrated to me, from Hafs ibn 'Asim, from Abu Sa'id ibn al-Mu'alla, who said: I was praying, and the Messenger of Allah (peace and blessings of Allah be upon him) called me, but I did not answer him until I had finished praying. Then I came to him, and he said: "What prevented you from coming to me?" I said: "O Messenger of Allah, I was praying." He said: "Did Allah not say: {O you who have believed, respond to Allah and to the Messenger when he calls you} [Al-Anfal: 24]?" Then he said: "I will surely teach you the greatest surah in the Qur'an—or of the Qur'an—before you leave the mosque." He said: Then he took my hand. When he intended to leave the mosque, I said: "O Messenger of Allah, did you not say: 'I will surely teach you the greatest surah in the Qur'an'?" He said: "Yes, [it is] 'All praise is due to Allah, Lord of the worlds' (Al-Hamdu lillahi Rabbil-'alamin). It is the seven oft-repeated verses and the Great Qur'an which I have been given."