Musnad Ahmad 27:490

'Abd al-Razzaq and Ibn Bakr narrated to us, they said: Ibn Jurayj informed us, he said: Ibn Shihab said: Salim ibn 'Abd Allah narrated to me, that 'Abd Allah ibn 'Umar used to walk in front of the funeral procession, and "the Messenger of Allah (peace and blessings of Allah be upon him), Abu Bakr, 'Umar, and 'Uthman used to walk in front of it."