Mustadrak al-Hakim — Hadith 3056

It is narrated from Sayyiduna Ubayy bin Ka'b (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Shall I not teach you a surah the like of which has not been revealed in the Torah, nor in the Injil, nor in the Zabur, nor in the Furqan (the Qur'an)?" I said: "Certainly (please teach me)." He (peace and blessings be upon him) said: "I hope that you will not leave through this door before you have learned it." Then the Messenger of Allah (peace and blessings be upon him) stood up and I went along with him. He (peace and blessings be upon him) began talking to me while my hand was in his (peace and blessings be upon him) blessed hand. I deliberately began to walk slowly, fearing that he (peace and blessings be upon him) might go out of the door (before completing what he had said). When I drew near the door, I said: "O Messenger of Allah! The surah that you promised me?" He (peace and blessings be upon him) asked: "What do you recite when you stand for prayer?" So I recited Fatihat al-Kitab. He (peace and blessings be upon him) said: "It is that surah, and it is the 'seven oft-repeated (verses)' and the 'Great Qur'an' that I have been given."
This hadith is sahih (authentic) on the condition of Muslim, but the two Shaykhs (Imam Bukhari and Muslim) did not record it.