Musnad Ahmad 27:158

'Abdah narrated to us, Muhammad ibn Ishaq narrated to us, from Muhammad ibn Ja'far ibn al-Zubayr, from 'Ubayd Allah ibn 'Abd Allah, from Ibn 'Umar, who said: I heard the Prophet, peace and blessings of Allah be upon him, being asked about water that is in the open wilderness, and what comes to it of beasts and predatory animals. The Prophet, peace and blessings of Allah be upon him, said: "When the water reaches the measure of two qullahs, it does not carry impurity."