Sahih Muslim 284a
Sahih Muslim · Book 2: The Book of Purification · The obligation to wash away urine and other impurities if they result in the Masjid, and the ground may be purified with water with no need to scrub it
Anas reported:
وَحَدَّثَنَا قُتَيْبَةُ بْنُ سَعِيدٍ ، حَدَّثَنَا حَمَّادٌ، - وَهُوَ ابْنُ زَيْدٍ - عَنْ ثَابِتٍ ، عَنْ أَنَسٍ ، أَنَّ أَعْرَابِيًّا، بَالَ فِي الْمَسْجِدِ فَقَامَ إِلَيْهِ بَعْضُ الْقَوْمِ فَقَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " دَعُوهُ وَلاَ تُزْرِمُوهُ " . قَالَ فَلَمَّا فَرَغَ دَعَا بِدَلْوٍ مِنْ مَاءٍ فَصَبَّهُ عَلَيْهِ .
A Bedouin urinated in the mosque. Some of the persons stood up (to reprimand him or to check him from doing so), but the Messenger of Allah (ﷺ) said: Leave him alone; don't interrupt him. He (the narrator) said: And when he had finished, he called for a bucket of water and poured it over.