Musannaf Ibn Abi Shayba 18:997

Abu Bakr narrated to us, he said: Ibn Idris narrated to us, from Husayn, from Amir, who said: He used to say regarding dogs: If a man approaches them while they are with the sheep and they bite him, then there is no liability upon him. And if they confront people on the road and they harm someone, then upon him is the liability.