Sahih Bukhari
Ablutions (Wudu)
Volume: 001 / Book Number: 004 / Hadith Number: 164
Narrated 'Abdullah bin 'Amr:
The Prophet remained behind us on a journey. He joined us while we were performing ablution for the 'Asr prayer which was over-due and we were just passing wet hands over our feet (not washing them thoroughly) so he addressed us in a loud voice saying twice orthriae, "Save your heels from the fire."