Deuteronomy 21:2-4

New King James Version

² then your elders and your judges shall go out and measure the distance from the slain man to the surrounding cities. ³ And it shall be that the elders of the city nearest to the slain man will take a heifer which has not been worked and which has not pulled with a ^(A)yoke. ⁴ The elders of that city shall bring the heifer down to a valley with flowing water, which is neither plowed nor sown, and they shall break the heifer’s neck there in the valley.