Sahih Muslim Book 2, Hadith Number 523.
Chapter : Wiping over the socks.
Abu Wa’il reported: Abu Musa inflicted extreme rigour upon himself in the matter of urination and urinated in a bottle and said: When the skin of anyone amongst the people of Israel was besmeared with urine, he cut that portion with a cutter. Hudhaifa said: I wish that your friend should not inflict such an extreme rigour. I and the Messenger of Allah (may peace be upon him) were going together till we reached the dumping ground of filth behind an enclosure. He stood up as one among you would stand up and he urinated, I tried to turn away from him, but he beckoned to me, so I went to him and I stood behind him, till he had relieved himself.