Deuteronomy 21:3

And it shall be, that the city which is next unto the slain man, even the elders of that city shall take an heifer, which hath not been wrought with, and which hath not drawn in the yoke;

Cross-reference

Numbers 19:2 also requires a heifer that has never been yoked, used for purification—the same ritual qualification appears in the unsolved murder atonement.