Sayyiduna Abu Sa'id bin al-Mu'alla (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) was in the mosque and I was praying, and he (peace and blessings be upon him) called me. After finishing the prayer I came to him (peace and blessings be upon him), and he (peace and blessings be upon him) said: "What prevented you from coming when I called you? Have you not heard this saying of Allah Almighty: 'O you who believe! Respond to Allah and the Messenger when he calls you to that which gives you life' [Surah al-Anfal: 24]? I will surely teach you the greatest surah in the Qur'an before I leave the mosque." I kept walking along with the Messenger of Allah (peace and blessings be upon him) until we were about to reach the door, and I said to myself that he (peace and blessings be upon him) must have forgotten, so I reminded him (peace and blessings be upon him). I said: "O Messenger of Allah! You said such and such." The Messenger of Allah (peace and blessings be upon him) said: "'All praise is for Allah, the Lord of all the worlds' [Surah al-Fatihah: 2]: it is the Seven Oft-Repeated (verses) and the Great Qur'an which I have been given."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3930