Sahih Bukhari Volume 8, Book 73, Hadith Number 54.

Narated By Anas bin Malik : A Bedouin urinated in the mosque and the people ran to (beat) him. Allah’s Apostle said, “Do not interrupt his urination (i.e. let him finish).” Then the Prophet asked for a tumbler of water and poured the water over the place of urine.