Musnad Ahmad 30:2222

'Affan narrated to us, he said: 'Abd al-Rahman ibn Ibrahim narrated to us, he said: al-'Ala' ibn 'Abd al-Rahman narrated to us, from his father, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) went out to Ubayy ibn Ka'b while he was praying, and said: "O Ubayy." He turned, but did not answer him. Then Ubayy prayed and made it brief, then he turned to the Messenger of Allah (peace and blessings of Allah be upon him) and said: "Peace be upon you, O Messenger of Allah." He said: "And upon you." He said: "What prevented you, O Ubayy, when I called you, from answering me?" He said: "O Messenger of Allah, I was in prayer." He said: "Do you not find in what Allah revealed to me: {Respond to Allah and to the Messenger when he calls you to that which gives you life} [Al-Anfal: 24]?" He said: "Yes, O Messenger of Allah, I will not do it again." He said: "Would you like me to teach you a Surah, the like of which has not been revealed in the Torah, nor in the Zabur, nor in the Injil, nor in the Furqan?" I said: "Yes, O Messenger of Allah." The Messenger of Allah (peace and blessings of Allah be upon him) said: "I hope that you will not go out of this door until you know it." He said: The Messenger of Allah (peace and blessings of Allah be upon him) took my hand and spoke to me, while I slowed my pace, fearing that he would reach the door before he finished the speech. When we drew near the door, I said: "O Messenger of Allah, what is the Surah you promised me?" He said: "What do you recite in the prayer?" He said: So I recited Umm al-Qur'an to him. The Messenger of Allah (peace and blessings of Allah be upon him) said: "By the One in Whose hand is my soul, Allah has not revealed in the Torah, nor in the Injil, nor in the Zabur, nor in the Furqan the like of it, and it is the seven oft-repeated verses."