Musnad Ahmad 27:1582

Sulayman ibn Dawud al-Hashimi narrated to us, Ibrahim ibn Sa'd informed us, the nephew of Ibn Shihab narrated to me, from Ibn Shihab, from Salim, from his father, who said: "The Messenger of Allah (peace and blessings of Allah be upon him), Abu Bakr, Umar, and Uthman used to walk in front of the funeral procession."