It is narrated from Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) was asked about water to which predators and animals come by turns, and he (peace and blessings be upon him) said: "When water is equal to two large jars (qullatayn), it does not take on filth (impurity) (it does not become impure)." Imam al-Shafi'i also narrated it in his book "al-Mabsut" through a trustworthy (thiqah) narrator, and that trustworthy narrator is without doubt Abu Usamah.