Musnad Ahmad 30:2408

Yahya ibn Sa'id narrated to us, he said: Yazid ibn Kaysan narrated to us, he said: Abu Hazim narrated to me, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Gather, for I will recite to you a third of the Qur'an." He said: So those who gathered, gathered. Then he (the Prophet) went out and recited: "Say, 'He is Allah, [who is] One.'" Then he went in. Some of us said to others: "This is news that has come to him from the heaven, and that is what caused him to go in." Then he went out and said: "I told you that I would recite to you a third of the Qur'an, and it is equivalent to a third of the Qur'an."