Hadith: It is not appropriate to use mosques for urinating or filth. Indeed, they are built for the remembrance of Allah and the recitation of the Qur’an

عن أنس رضي الله عنه مرفوعاً: «إن هذه المساجد لا تَصْلُحُ لشيء من هذا البَول ولا القَذَر، إنما هي لِذِكْر الله عز وجل، والصلاة، وقراءة القرآن» أو كما قال رسول الله صلى الله عليه وسلم .
[صحيح] - [رواه مسلم]

Anas (may Allah be pleased with him) reported that the Prophet (may Allah's peace and blessings be upon him) said: "It is not appropriate to use mosques for urinating or filth. Indeed, they are built for the remembrance of Allah and the recitation of the Qur’an." Or he said words similar to this.
Sahih/Authentic. - [Muslim]

Explanation

There is a story behind this Hadīth which Anas (may Allah be pleased with him) reported. He said: Once we were in the mosque with the Messenger of Allah (may Allah's peace and blessings be upon him) when a Bedouin came in and started urinating in the mosque. The Companions of the Messenger of Allah started to shout at him to stop. The Messenger of Allah said: "Do not interrupt him; leave him alone." So they left him until he finished urinating. Then the Messenger of Allah called him over and gently told him that these mosques are not built for urinating or any kind of filth; rather, they are built for prayer, reading the Qur’an, and remembrance of Allah. It is the duty of the believer to respect the houses of Allah. He should never throw anything filthy or harmful inside them nor raise his voice unduly, and be at his best behavior inside mosques because they are the houses of Allah.