Maliks Muwatta Book 3, Hadith Number 39.

Yahya related to me from Malik from al-Ala ibn Abd ar-Rahman ibn Yaqub that Abu Said, the mawla of Amir ibn Kuraz told him that the Messenger of Allah, may Allah bless him and grant him peace, called to Ubayy ibn Kab while he was praying. When Ubayy had finished his prayer he joined the Messenger of Allah, may Allah bless him and grant him peace, and the Messenger of Allah put his hand upon his hand, and he was intending to leave by the door of the mosque, so the Messenger of Allah, may Allah bless him and grant him peace, said, “I hope that you will not leave the mosque until you know a sura whose like Allah has not sent down in the Tawrah nor in the Injil nor in the Qur’an.” Ubayy said, “I began to slow down my pace in the hope of that. Then I said, ‘Messenger of Allah, the sura you promised me!’ He said, ‘What do you recite when you begin the prayer?’ I recited the Fatiha (Sura 1) until I came to the end of it, and the Messenger of Allah, may Allah bless him and grant him peace, said, ‘It is this sura, and it is the “seven oft-repeated” and the Great Qur’an which I was given.'”