Musnad Ahmad 1034:6

Sufyan ibn 'Uyaynah narrated to us, from Za'idah, from 'Abd al-Malik ibn 'Umayr, from Rib'i ibn Hirash, from Hudhayfah, that the Prophet, may Allah bless him and grant him peace, said: "Follow the example of the two who are after me: Abu Bakr and 'Umar."