Sayyiduna Abu Sa'id bin al-Mu'alla (may Allah be pleased with him) said: I was praying when the Messenger of Allah (peace and blessings be upon him) called me, and I did not answer him. When I had finished the prayer, he (peace and blessings be upon him) said: "What prevented you from answering me when I called you? Allah Almighty says: O you who believe, respond to Allah and the Messenger when he calls you to that which gives you life. [Surah al-Anfal: 24]" Then he said: "Before you leave the mosque, I will teach you a surah of the Qur'an." He said: I walked with the Messenger of Allah (peace and blessings be upon him) until he (peace and blessings be upon him) was about to leave the mosque, and I reminded him. He (peace and blessings be upon him) said: "It is Surah al-Fatihah, which is the seven oft-repeated verses (al-Sab' al-Mathani) and the Mighty Qur'an that I have been given."
Hadith Referenceالمعجم الكبير للطبراني / مسند من يعرف بالكنى / 18972