Musnad Ahmad 30:1574

Muhammad ibn Abdullah ibn al-Zubayr narrated to us, Aban—meaning Ibn Abdullah al-Bajali—narrated to us, a freed slave of Abu Hurayrah narrated to me, he said: I heard Abu Hurayrah saying: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Bring me water for ablution." So I brought him water for ablution. He cleaned himself (istinja), then he put his hand into the soil and wiped it, then he washed it, then he performed ablution and wiped over his leather socks. I said: "O Messenger of Allah, you did not wash your feet." He said: "I inserted them while they were in a state of purity."