كتاب الطهارة
2
The Book of Purification
(30)
Chapter: 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
(30)
باب وُجُوبِ غَسْلِ الْبَوْلِ وَغَيْرِهِ مِنَ النَّجَاسَاتِ إِذَا حَصُلَتْ فِي الْمَسْجِدِ وَأَنَّ الأَرْضَ تَطْهُرُ بِالْمَاءِ مِنْ غَيْرِ حَاجَةٍ إِلَى حَفْرِهَا

Anas reported:

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.
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 ‏.‏
Reference : Sahih Muslim 284a
In-book reference : Book 2, Hadith 125
USC-MSA web (English) reference : Book 2, Hadith 557
  (deprecated numbering scheme)