Maliks Muwatta Book 20, Hadith Number 157.
Section : What to Do with Sacrificial Animals (Hadys) if They get Injured or Stray.
Yahya related to me from Malik that Ibn Shihab said, “If someone dedicates an animal as compensation, or for a vow, or as the sacrifice for tamattu, and misfortune befalls it on the road, he must provide a substitute.”