Hadith: If there is enough water to fill two pitchers, then it will bear no impurity

عن عبد الله بن عمر رضي الله عنهما قال: سُئِلَ رسول الله صلى الله عليه وسلم عن الماء وما يَنُوبُهُ من الدواب والسِّبَاعِ، فقال صلى الله عليه وسلم : «إذا كان الماء قُلَّتين لم يحمل الخَبَثَ».
[صحيح] - [رواه أبو داود وابن ماجه والترمذي والنسائي والدارمي وأحمد]

ʿAbdullāh ibn ‘Umar (may Allah be pleased with him) reported: The Messenger of Allah (may Allah’s peace and blessings be upon him) was asked about water that was frequented by animals and beasts of prey. He said: "If there is enough water to fill two pitchers, then it will bear no impurity."
Sahih/Authentic. - [Ibn Maajah]

Explanation

The Prophet (may Allah's peace and blessings be upon him) explains that a large amount of water does not become impure by mere contact with filth, so long as none of its properties changes. A small amount of water, however, gets impure when it comes in contact with filth, which will definitely change it. If a large amount of water is exposed to filth and one of its properties changes, it will become impure, even if the amount is enough to fill two pitchers. Mentioning that when being asked about the leftover of animals and beasts of prey indicates that their leftover is not pure, unless water is in a large amount and its properties do not change upon contact with the mouths of animals.