Deuteronomy 21:3-4

3and the city that is nearest unto him that is slain, even the elders of that city shall take a heifer that hath not been wrought with, that hath not drawn in the yoke;

4and the elders of that city shall bring down the heifer unto an ever-flowing watercourse, which is not tilled, nor is it sown, and shall break the heifer's neck there in the watercourse;