Abdullah bin Abdullah bin Umar narrates from his father (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) was asked about water which beasts of prey and animals come to. He (peace and blessings be upon him) said: "When the water amounts to two qullahs (large jars), it does not carry impurity (that is, it does not become impure)."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 1231