Musnad Ahmad 5:142

Ya'qub narrated to us, my father narrated to us, from Ibn Ishaq, who said: Muhammad ibn Ka'b al-Qurazi mentioned, from al-Harith ibn 'Abd Allah al-A'war, who said: I said: I will surely come to the Commander of the Faithful and ask him about what I heard this evening. He said: So I came to him after the night prayer and entered upon him, and he mentioned the hadith. He said: Then he said: I heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: "Gabriel came to me and said: 'O Muhammad, your nation will differ after you.' He said: So I said to him: 'Then where is the way out, O Gabriel?' He said: He said: 'The Book of Allah the Exalted; by it Allah breaks every tyrant. Whoever holds fast to it is saved, and whoever abandons it perishes' — [he said it] twice — 'a decisive word, and it is not a jest. Tongues do not distort it, and its wonders do not cease. In it is the news of what was before you, the judgment of what is between you, and the report of what will be after you.'"