Abu Dawud said: A dumb animal means an animal which is free and has not tether, and there is no one (as a watchman) with it. It causes harm by day and not by night.
In-book : Book 41, Hadith 100
English translation : Book 40, Hadith 4576
41. Types of Blood-Wit (Kitab Al-Diyat)
باب الْعَجْمَاءُ وَالْمَعْدِنُ وَالْبِئْرُ جُبَارٌ