Sahih Muslim Book 2, Hadith Number 522.

Hudhaifa reported: I was with the Apostle of Allah (may peace be upon him) when he came to the dumping ground of filth belonging to a particular tribe. He urinated while standing, and I went aside. He (the Holy Prophet) asked me to come near him and I went so near to him that I stood behind his heels. He then performed ablution and wiped over his socks.