"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 carry filth.'"[1] [1]It comes with some explanation in Sunan At Timidhi 'Abdah (one of the narrators) said: "Muhammad bin Ishaq said: 'A Qullah refers to Jirar (These are two nouns describing large casks that are used to hold water), and a Qullah is the thing that drinking water is held in."' At-Tirmidhi said: "This is the saying of Ash Shafa'i, Ahmad and Ishaq. They say that when the water is two Qullahs then nothing makes it impure, as long as it does not change its smell, and its taste. And they say, it is approximately fifty Qirbahs (waterskins)."  

سُئِلَ رَسُولُ اللَّهِ ﷺ عَنِ الْمَاءِ وَمَا يَنُوبُهُ مِنَ الدَّوَابِّ وَالسِّبَاعِ فَقَالَ إِذَا كَانَ الْمَاءُ قُلَّتَيْنِ لَمْ يَحْمِلِ الْخَبَثَ