SAHIH BUKHARI

Volume 1, Book 3, Number 96:

Narrated 'Abdullah bin 'Amr:

Once Allah's Apostle remained behind us in a journey. He joined us while we were performing ablution for the 'Asr prayer which was over-due. We were just passing wet hands over our feet (not washing them properly) so the Prophet addressed us in a loud voice and said twice or thrice, "Save your heels from the fire."