Musnad Ahmad 32:192

Yahya ibn Sa'id narrated to us, from Yahya ibn Sa'id, who said: I heard Anas ibn Malik saying: A Bedouin entered the mosque during the time of the Messenger of Allah, may Allah bless him and grant him peace, and urinated. The people forbade him. The Messenger of Allah, may Allah bless him and grant him peace, said: "Leave him." And he ordered that water be poured over it, or that water be poured out upon it.