Musannaf Ibn Abi Shayba 35:46

Abu Usama narrated to us, from al-Walid ibn Kathir, from Muhammad ibn Ja'far ibn al-Zubayr, from Ubayd Allah ibn Abd Allah ibn Umar, from his father, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "When the water reaches two qullas, it does not carry impurity." And he mentioned that Abu Hanifa said: "The water becomes impure."