Musnad Ahmad 30:2473

Yahya narrated to us, from Ibn Abi Dhi'b, who said: Salih, the freed slave of al-Taw'amah, narrated to me, saying: I heard Abu Hurairah, from the Prophet, peace and blessings of Allah be upon him, who said: "Whoever washes a dead person, let him perform ghusl (a full ritual bath)."