Musnad Abu Ya'la al-Mawsili 123:4

Abu Khaythama narrated to us, Yahya ibn Sa'id al-Qattan narrated to us, from Yahya ibn Sa'id—Abu Khaythama said: meaning the Ansari—he said: I heard Anas ibn Malik, may Allah be pleased with him, say: A Bedouin urinated in the mosque, and his companions wanted to stop him. The Messenger of Allah, peace and blessings of Allah be upon him, said: "Leave him." Then he ordered water, and it was poured over it.