Sunan an-Nasa'i 53
It was narrated from Anas that a Bedouin urinated in the Masjid, and some of the people went after him, but the Messenger of Allah (ﷺ) said:
"Leave him and do not restrain him." When he had finished he called for a bucket (of water) and poured it over it. [1]

Abu 'Abdur-Rahman (An-Nasa'i) said: "Meaning: 'Do not interrupt him.'"

[1]The author will cite this narration again in No. 330 as a possible proof for setting the minimum, since it mentions "a bucket" as if this is the minium amount required.
أَخْبَرَنَا قُتَيْبَةُ، قَالَ حَدَّثَنَا حَمَّادٌ، عَنْ ثَابِتٍ، عَنْ أَنَسٍ، أَنَّ أَعْرَابِيًّا، بَالَ فِي الْمَسْجِدِ فَقَامَ إِلَيْهِ بَعْضُ الْقَوْمِ فَقَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏
"‏ دَعُوهُ لاَ تُزْرِمُوهُ ‏"‏ ‏.‏ فَلَمَّا فَرَغَ دَعَا بِدَلْوٍ فَصَبَّهُ عَلَيْهِ ‏.‏ قَالَ أَبُو عَبْدِ الرَّحْمَنِ يَعْنِي لاَ تَقْطَعُوا عَلَيْهِ ‏.‏