Abu Dawud Book 34, Hadith Number 4576.
Chapter : No retaliation is payable for a wound caused by a dumb animal for a mine and for a well.
Narated By Abu Hurairah : The Apostle of Allah (may peace be upon him) said: No retaliation is payable for a wound caused by a dumb animal, for a mine, and for a well. On the treasure found buried in the land there is a fifth.
Abu Dawud said : A dumb animal means an animal which is free and has no tether, and there is no one (as a watchman) with it. It causes harm by day and not by night.