Musnad Ahmad 32:1041

Bahz narrated to us, 'Ikrimah ibn 'Ammar narrated to us, Ishaq ibn 'Abdullah ibn Abi Talhah al-Ansari narrated to us, from his paternal uncle Anas ibn Malik, who said: The Messenger of Allah (may Allah bless him and grant him peace) was sitting in the mosque and his companions were with him, when a Bedouin came and urinated in the mosque. His companions said: "Mah, mah!" (an expression of disapproval). The Messenger of Allah (may Allah bless him and grant him peace) said: "Do not interrupt him; leave him." Then he called him and said to him: "Indeed, these mosques are not suitable for anything of filth, urine, or excrement," or as the Messenger of Allah (may Allah bless him and grant him peace) said: "They are only for the recitation of the Qur'an, the remembrance of Allah, and prayer." Then the Messenger of Allah (may Allah bless him and grant him peace) said to a man from the people: "Stand up and bring us a bucket of water, and pour it over it." So he brought him a bucket of water and poured it over it.