Sunan Ibn Majah, Chapter: The Chapters of Purification and its Sunnah

"A Bedouin came to the Prophet (saw) and said: 'O Allah, have mercy on me and Muhammad (SAW), and do not allow anyone else to share in Your mercy.' The Prophet (saw) said: 'You have placed restrictions on something that is vast, woe to you!' Then (the Bedouin) spread his legs and urinated, and the Companions of the Prophet (saw) told him to stop, but the Messenger of Allah (saw) said: 'Let him be,' then he called for a vessel of water and poured it over (the urine)." (Hasan)