Musnad Abu Ya'la al-Mawsili 153:50

Mus'ab ibn 'Abd Allah al-Zubayri narrated to us, Ibrahim ibn Sa'd narrated to us, from the nephew of Ibn Shihab, from al-Zuhri, from Salim, from his father: That 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.