It was narrated from Abu Sa'eed Al-Khudri that:
The Prophet was asked about the water basins located between Makkah and Al-Madinah, which were visited by wild animals, dogs and donkeys, and about using them for means of purification. He said: "Whatever they (the animals) have carried in their bellies is for them, and whatever is left over is for us, and is pure."
English reference: Vol. 1, Book 1, Hadith 519
Arabic reference: Book 1, Hadith 561