Musnad Ahmad 27:355

Yazid ibn Harun narrated to us, Muhammad ibn Ishaq informed us, from Muhammad ibn Ja'far ibn al-Zubayr, from Ubayd Allah ibn Abd Allah ibn Umar, from his father Abd Allah ibn Umar, who said: I heard the Messenger of Allah, may Allah bless him and grant him peace, while he was being asked about water that is in the open country of the land, and what comes to it of beasts and predatory animals. The Prophet, may Allah bless him and grant him peace, said: "When the water reaches two qullahs, nothing makes it impure."