It was narrated from 'Ubaidullah bin 'Abdullah bin 'Umar that his father said:
"I heard the Messenger of Allah being asked about water in the wilderness that is frequented by beasts and predators. The Messenger of Allah said: "If the water reaches the amount of two Qullah, nothing can make it impure (Najis).'" (Sahih)Another chain with similar wording. (ES)
English reference (ES): Vol. 1, Book 1, Hadith 517 (ES)
Arabic reference (ES): Book 1, Hadith 557 (ES)