It was narrated that Abu Hurairah said:
"A Bedouin entered the mosque when the Messenger of Allah was sitting there, and (the man) said: 'O Allah, forgive me and Muhammed, and do not forgive anyone else with us.' The Messenger of Allah smiled and said: 'You have placed restrictions on something that is vast.' Then the Bedouin turned away, went to a corner of the mosque, spread his legs and began to urinate. After he had a better understanding, the Bedouin said: 'He got up and came to me, and may my father and mother be ransomed for him, he did not rebuke me nor revile me. He said: "This mosque is not for urinating in. Rather it is built for the remembrance of Allah and prayer.'" Then he called for a large vessel of water and poured it over the place where he had urinated."
English reference: Vol. 1, Book 1, Hadith 529
Arabic reference: Book 1, Hadith 572