Anas bin Malik (may Allah be pleased with him) said: I saw a Bedouin urinating in the mosque, and the Prophet (peace and blessings be upon him) said: "Leave him." When he had finished, he (peace and blessings be upon him) had water brought and poured it over it.
(b) In another narration: He (peace and blessings be upon him) ordered one of the people, who brought a bucket of water, and he (peace and blessings be upon him) poured it over it.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4238