Musnad Abu Ya'la al-Mawsili 155:16

Al-Hudhayl ibn Ibrahim al-Jumani narrated to us, Uthman ibn Abd al-Rahman al-Zuhri narrated to us, from al-Zuhri, from Sa'id ibn al-Musayyib, from Abu Hurayrah, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "This Ummah will act for a period by the Book of Allah, then it will act for a period by the Sunnah of the Messenger of Allah, then it will act by opinion (al-ra'y). So when they act by opinion, they have gone astray and led others astray."