(30) باب وُجُوبِ غَسْلِ الْبَوْلِ وَغَيْرِهِ مِنَ النَّجَاسَاتِ إِذَا حَصُلَتْ فِي الْمَسْجِدِ وَأَنَّ الأَرْضَ تَطْهُرُ بِالْمَاءِ مِنْ غَيْرِ حَاجَةٍ إِلَى حَفْرِهَا
Anas b. Malik reported:
While we were in the mosque with Allah's Messenger (ﷺ), a desert Arab came and stood up and began to urinate in the mosque. The Companions of Allah's Messenger (ﷺ) said: Stop, stop, but the Messenger of Allah (ﷺ) said: Don't interrupt him; leave him alone. They left him alone, and when he finished urinating, Allah's Messenger (ﷺ) called him and said to him: These mosques are not the places meant for urine and filth, but are only for the remembrance of Allah, prayer and the recitation of the Qur'an, or Allah's Messenger said something like that. He (the narrator) said that he (the Holy Prophet) then gave orders to one of the people who brought a bucket of water and poured It over.
Reference: Sahih Muslim 285
In-book reference: Book 2, Hadith 127
USC-MSA web (English) reference(deprecated numbering scheme): Book 2, Hadith 559