Musnad Ahmad 32:1424

Yunus ibn Muhammad narrated to us, Hammad—meaning ibn Zayd—narrated to us, from Thabit, I do not know it except from Anas, that a Bedouin came to the mosque of the Prophet, may Allah bless him and grant him peace, and urinated in it. The people stood up toward him, but the Messenger of Allah, may Allah bless him and grant him peace, said: "Leave him, do not interrupt him." Then he called for water and poured it over it.