Sayyiduna Abdullah bin Umar (may Allah be pleased with him) says that the Prophet (peace and blessings be upon him) was asked about water that is in open desert land which animals and beasts of prey come to, and the Messenger of Allah (peace and blessings be upon him) said: "When the water is the amount of two qullahs (large jars), it does not carry impurity (that is, it does not become impure)."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 1241