Sahih Bukhari Volume 7, Book 72, Hadith Number 750.

Narrated By Abu Juhaifa : I came to the Prophet while he was inside a red leather tent, and I saw Bilal taking the remaining water of the ablution of the Prophet, and the people were taking of that water and rubbing it on their faces; and whoever could not get anything of it, would share the moisture of the hand of his companion (and then rub it on his face).