From Sahih Bukhari (Hadith 203):
Narrated Al-Mughira bin Shu`ba:Once Allah's Messenger (ﷺ) went out to answer the call of nature and I followed him with a tumbler containing water, and when he finished, I poured water and he performed ablution and passed wet hands over his Khuffs (socks made from thick fabric or leather)
Reference: Bukhari 203