Narrated `Abdullah bin `Amr: Once the Prophet remained behind
us in a journey. He joined us while we were performing
ablution for the prayer which was over−due. We were just
passing wet hands over our feet (and not washing them
properly) so the Prophet addressed us in a loud voice and said
twice or thrice: "Save your heels from the fire."
(Sahih Bukhari chapter Book of Knowledge kitabul ilm)