Sahih Bukhari Volume 001, Book 004, Hadith Number 182.

Narated By Al-Mughira bin Shu'ba : I was in the company of Allah's Apostle on one of the journeys and he went out to answer the call of nature (and after he finished) I poured water and he performed ablution; he washed his face, forearms and passed his wet hand over his head and over the two Khuff, (leather socks).