Deuteronomy 21:3-4

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

4and the elders of that city shall bring down the heifer unto a valley with running water, which is neither plowed nor sown, and shall break the heifer’s neck there in the valley.