Musnad Ahmad 30:2181

Muhammad ibn Ja'far narrated to us, he said: Shu'bah narrated to us, from Muhammad ibn Ziyad, that he said: Abu Hurairah used to come upon the people while they were performing ablution at the place of purification, and he would say to them: "Perform the ablution thoroughly, perform the ablution thoroughly, for I heard Abu al-Qasim (peace and blessings of Allah be upon him) say: 'Woe to the heels from the Fire.'"