Musnad Ahmad 155:1

Muhammad ibn Ja‘far narrated to us, Shu‘bah narrated to us, from Khubayb ibn ‘Abd al-Rahman, from Hafs ibn ‘Asim, from Abu Sa‘id ibn al-Mu‘alla, who said: I was praying, and the Messenger of Allah (may Allah bless him and grant him peace) passed by me and called me, but I did not come to him until I had finished praying. Then I came to him, and he said: "What prevented you from coming to me?" I said: "I was praying." He said: "Did Allah, Blessed and Exalted is He, not say: {O you who have believed, respond to Allah and to the Messenger when he calls you to that which gives you life} [Al-Anfal: 24]?" Then he said: "Shall I not teach you the greatest Surah in the Qur’an before I leave the mosque?" He said: Then the Messenger of Allah (may Allah bless him and grant him peace) went to leave, so I reminded him. He said: "Al-Hamdu lillahi Rabbil-‘Alamin (Praise be to Allah, Lord of the Worlds) is the seven oft-repeated verses and the Great Qur’an which I have been given."