Musnad Abu Ya'la al-Mawsili 110:231

Zuhayr narrated to us, Isma'il ibn Abi Uways narrated to us, my father narrated to me, from Thawr ibn Zayd, from 'Ikrimah, from Ibn 'Abbas, that he said: A Bedouin came to the Prophet, may Allah bless him and grant him peace, and pledged allegiance to him in the mosque. Then he turned away, stood up, spread his legs, and urinated. The people intended to [stop] him, but the Prophet, may Allah bless him and grant him peace, said: "Do not interrupt the man's urination." Then he called for him and said: "Are you not a Muslim?" He said: "Yes." He said: "What compelled you to urinate in the mosque?" He said: "By the One who sent you with the truth, I did not think it was anything but one of the [open] grounds, so I urinated in it." The Prophet, may Allah bless him and grant him peace, then ordered a bucket of water, and it was poured over his urine.