Muwatta Malik 858
موطأ مالك#858SahihYahya related to me from Malik from Nafi that Abdullah ibn Umar said, "If someone dedicates an animal and then it goes astray or dies, he should provide a substitute, if it was for a vow. If, however, it was voluntary, then he can either provide a substitute for it or not, as he wishes
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Salim al-HilaliMauquf Sahih
In-book reference: 20:154