Ubaydullah bin Abdullah bin Umar says: My father narrated to me (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "When the water amounts to two qullahs (large jars), it does not become impure."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 1243