Musnad Ahmad 30:2142

'Affan narrated to us, he said: Hammad narrated to us, he said: Muhammad ibn Ziyad narrated to us, that Abu Hurairah saw a man with unwashed spots on his feet, so he said: "Perform wudu well, for I heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: 'Woe to the heels from the Fire.'"