Musnad Ahmad 27:1794

Hajjaj narrated to us, he said: I read to Ibn Jurayj, Ziyad ibn Sa'd narrated to me, that Ibn Shihab said: Salim narrated to me, from Abdullah ibn Umar, that he 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 (may Allah be pleased with them) used to walk in front of it.