Musnad Ahmad 790:183

Mu'ammal narrated to us, Hammad ibn Zayd narrated to us, from Ali ibn Zayd, from al-Hasan, from Imran ibn Husayn, who said: The Quran was revealed, and the Messenger of Allah, peace and blessings of Allah be upon him, established the Sunnahs. Then he said: "Follow us, for by Allah, if you do not do so, you will go astray."