Sahih Bukhari Volume 001, Book 004, Hadith Number 164.

Narated By 'Abdullah bin 'Amr : The Prophet remained behind us on a journey. He joined us while we were performing ablution for the 'Asr prayer which was over-due and we were just passing wet hands over our feet (not washing them thoroughly) so he addressed us in a loud voice saying twice orthriae, "Save your heels from the fire."