Musnad Ahmad 30:2160

'Affan narrated to us, he said: Hammad ibn Salamah narrated to us, he said: Muhammad ibn Ziyad informed us that Abu Hurairah saw a man with unwashed spots on his heels, so he said: "Perform ablution well, for I heard Abu al-Qasim (peace and blessings of Allah be upon him) say: 'Woe to the heels from the Fire.'"