Musnad Ahmad 626:13

Ya'qub narrated to us, my father narrated to us, from Ibn Ishaq, who said: I had memorized from many of our scholars in Medina that Muhammad ibn 'Amr ibn Hazm used to narrate from al-Mughirah hadiths, among them that he narrated to him that he heard the Prophet, peace and blessings of Allah be upon him, saying: "Whoever washes a dead person, let him perform ghusl (a ritual bath)."