Musnad Abu Ya'la al-Mawsili 153:105

Ya'qub ibn Ibrahim al-Bakri narrated to us, Abu 'Asim narrated to us, from Ibn Jurayj, Ibn Shihab informed me, Salim ibn 'Abdullah informed me, from 'Abdullah ibn 'Umar, that he used to walk in front of the funeral procession. And the Messenger of Allah (may Allah bless him and grant him peace), Abu Bakr, 'Umar, and 'Uthman used to walk in front of it.