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 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."
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."