Deuteronomy 21:3-4

3It shall be that the city which is nearest to the slain man, that is, the elders of that city, shall take a heifer of the herd, which has not been worked and which has not pulled in a yoke;

4and the elders of that city shall bring the heifer down to a valley with running water, which has not been plowed or sown, and shall break the heifer’s neck there in the valley.