Deuteronomy 21:2

1If someone is found slain in the land which the LORD your God gives you to possess, lying in the field, and it isn’t known who has struck him,

2then your elders and your judges shall come out, and they shall measure to the cities which are around him who is slain.

3It shall be that the elders of the city which is nearest to the slain man shall take a heifer of the herd, which hasn’t been worked with and which has not drawn in the yoke.

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