Sunan an-Nasa'i 328
It was narrated from 'Ubaidullah bin 'Abdullah bin 'Umar that his father said:
"The Messenger of Allah (ﷺ) was asked about water and how some animals and carnivorous beasts might drink from it. He said: 'If the water is more than two Qullahs, it will not become filthy.'"
أَخْبَرَنَا الْحُسَيْنُ بْنُ حُرَيْثٍ الْمَرْوَزِيُّ، قَالَ حَدَّثَنَا أَبُو أُسَامَةَ، عَنِ الْوَلِيدِ بْنِ كَثِيرٍ، عَنْ مُحَمَّدِ بْنِ جَعْفَرِ بْنِ الزُّبَيْرِ، عَنْ عُبَيْدِ اللَّهِ بْنِ عَبْدِ اللَّهِ بْنِ عُمَرَ، عَنْ أَبِيهِ، قَالَ سُئِلَ رَسُولُ اللَّهِ صلى الله عليه وسلم عَنِ الْمَاءِ وَمَا يَنُوبُهُ مِنَ الدَّوَابِّ وَالسِّبَاعِ فَقَالَ ‏
"‏ إِذَا كَانَ الْمَاءُ قُلَّتَيْنِ لَمْ يَحْمِلِ الْخَبَثَ ‏"‏ ‏.‏