These mosques are not appropriate for anything of this kind of urine or filth. They are only for the remembrance of Allah Almighty, prayer, and recitation of the Qur’an

عَنْ أَنَسِ بْنِ مَالِكٍ رَضيَ اللهُ عنه قَالَ: بَيْنَمَا نَحْنُ فِي الْمَسْجِدِ مَعَ رَسُولِ اللهِ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ إِذْ جَاءَ أَعْرَابِيٌّ، فَقَامَ يَبُولُ فِي الْمَسْجِدِ، فَقَالَ أَصْحَابُ رَسُولِ اللهِ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ: مَهْ مَهْ، قَالَ: قَالَ رَسُولُ اللهِ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ: «لَا تُزْرِمُوهُ، دَعُوهُ» فَتَرَكُوهُ حَتَّى بَالَ، ثُمَّ إِنَّ رَسُولَ اللهِ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ دَعَاهُ فَقَالَ لَهُ: «إِنَّ هَذِهِ الْمَسَاجِدَ لَا تَصْلُحُ لِشَيْءٍ مِنْ هَذَا الْبَوْلِ، وَلَا الْقَذَرِ إِنَّمَا هِيَ لِذِكْرِ اللهِ عَزَّ وَجَلَّ، وَالصَّلَاةِ وَقِرَاءَةِ الْقُرْآنِ» أَوْ كَمَا قَالَ رَسُولُ اللهِ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ قَالَ: فَأَمَرَ رَجُلًا مِنَ الْقَوْمِ فَجَاءَ بِدَلْوٍ مِنْ مَاءٍ فَشَنَّهُ عَلَيْهِ.

Anas ibn Mālik (may Allah be pleased with him) reported: While we were in the mosque with the Messenger of Allah (may Allah's peace and blessings be upon him), a Bedouin came and began to urinate in the mosque. The Companions of the Messenger of Allah (may Allah's peace and blessings be upon him) said: Stop! Stop! He said: The Messenger of Allah (may Allah's peace and blessings be upon him) said: "Do not interrupt him; let him." So they left him until he finished urinating. Then the Messenger of Allah (may Allah's peace and blessings be upon him) called him and said to him: "These mosques are not appropriate for anything of this kind of urine or filth. They are only for the remembrance of Allah Almighty, prayer, and recitation of the Qur’an" or as the Messenger of Allah (may Allah's peace and blessings be upon him) said. He said: Then he ordered a man from among the people who brought a bucket of water and poured it over the affected area.

Source · Agreed upon

From the Hadith Encyclopedia (hadeethenc.com).