SAHIH BUKHARI

Volume 1, Book 3, Number 57:

Narrated 'Abdullah bin 'Amr:

Once the Prophet remained behind us in a journey. He joined us while we were performing ablu- tion 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."