Musnad Ahmad 30:2426

Yahya narrated to us, from Shu'bah, who said: Muhammad ibn Ziyad and Hajjaj narrated to us, who said: Shu'bah narrated to us, from Muhammad ibn Ziyad, who said: Abu Hurairah used to pass by us while we were performing ablution from the water container, and he would say to us: "Perform the ablution thoroughly, for I heard the Messenger of Allah (peace and blessings of Allah be upon him) say: 'Woe to the heels from the Fire.'" Hajjaj said: "The heel."