Volumn 001, Book 003, Hadith Number 057.
Narated By '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."