Musnad Ahmad 1034:2

Hushaym narrated to us, he said: Al-A'mash informed us, from Abu Wa'il, from Hudhayfah ibn al-Yaman, who said: "I saw the Messenger of Allah (peace and blessings of Allah be upon him) come to a trash heap of some people, and he urinated while standing. Then he called for water, so I brought it to him, and he performed ablution and wiped over his leather socks."