Musnad Ahmad 30:653

Yunus narrated to us, Aban narrated to us, from Yahya ibn Abi Kathir, from a man from Banu Layth, from Abu Ishaq, that he heard Abu Hurayrah saying: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever washes a dead person, let him perform ghusl (ritual bath)."