Deuteronomy 21:4

4 and 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.

Deuteronomy 21:4 In-Context

2 then your elders and your judges shall go out and measure the distance to the cities which are around the slain one.
3 "It 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;
4 and 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.
5 "Then the priests, the sons of Levi, shall come near, for the LORD your God has chosen them to serve Him and to bless in the name of the LORD ; and every dispute and every assault shall be settled by them.
6 "All the elders of that city which is nearest to the slain man shall wash their hands over the heifer whose neck was broken in the valley;
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