Musnad Abu Ya'la al-Mawsili 118:73

Harun ibn Abdullah narrated to us, Muhammad ibn Bakr al-Barsani narrated to us, Yunus ibn Yazid al-Ayli narrated to us, al-Zuhri narrated to me, from Anas ibn Malik, who said: The Messenger of Allah (may Allah bless him and grant him peace), Abu Bakr, Umar, and Uthman used to walk in front of the funeral procession.