Sunan Ibn Majah 519
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."
حَدَّثَنَا أَبُو مُصْعَبٍ الْمَدَنِيُّ، حَدَّثَنَا عَبْدُ الرَّحْمَنِ بْنُ زَيْدِ بْنِ أَسْلَمَ، عَنْ أَبِيهِ، عَنْ عَطَاءِ بْنِ يَسَارٍ، عَنْ أَبِي سَعِيدٍ الْخُدْرِيِّ، أَنَّ النَّبِيَّ ـ صلى الله عليه وسلم ـ سُئِلَ عَنِ الْحِيَاضِ الَّتِي بَيْنَ مَكَّةَ وَالْمَدِينَةِ تَرِدُهَا السِّبَاعُ وَالْكِلاَبُ وَالْحُمُرُ وَعَنِ الطَّهَارَةِ مِنْهَا فَقَالَ ‏
"‏ لَهَا مَا حَمَلَتْ فِي بُطُونِهَا وَلَنَا مَا غَبَرَ طَهُورٌ ‏"‏ ‏.‏