Narrated 'Abdullah bin 'Amr (Radi-Allahu 'anhu):
Once the Prophet (Sallallahu 'Alaihi Wa Sallam) remained behind us in a
journey. He joined us whilewe 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 (Sallallahu 'Alaihi Wa Sallam)
addressed us in a loud voice and said twice or thrice: "Save your heels
from the fire."
Bukhari Vol. 1 : No. 57