Musnad Ahmad 27:1793

Hajjaj narrated to us, Layth narrated to us, Aqil ibn Khalid narrated to me, from Ibn Shihab, that Salim ibn Abdullah ibn Umar informed him that Abdullah ibn Umar used to walk in front of the funeral procession, "and that the Messenger of Allah (may Allah bless him and grant him peace) used to walk in front of it," as did Abu Bakr, Umar, and Uthman (may Allah be pleased with them).