My father told me that the Messenger of Allah ﷺ said, "When water reaches two qullahs, it does not become impure."
حدثنا موسى بن إسمعيل حدثنا حماد أخبرنا عاصم بن المنذر
حدثني أبي أن رسول الله ﷺ قال إذا كان الماء قلتين فإنه لا ينجس
[Its isnad is hasan] Ibn Majah narrated it in Sunan Ibn Majah, Purification, chapter on the amount of water that is not rendered impure, hadith 518, from the hadith of Hammad bin Salama, at length.