Musnad Ahmad 922:1

Yahya ibn Sa'id narrated to us, from Ajlah, Abdullah ibn Abd al-Rahman ibn Abza narrated to us, from his father, from Ubayy ibn Ka'b, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Indeed, Allah commanded me to recite the Quran to you." He (Ubayy) said: "Did my Lord name me to you?" He (the Prophet) said: "(Say: In the bounty of Allah and in His mercy - in that let them rejoice)." That is how Ubayy recited it.