Deuteronomy 21:6-9

6And all the elders of that city nearest to the slain man shall wash their hands over the heifer whose neck was broken in the valley,

7and they shall testify, 'Our hands did not shed this blood, nor did our eyes see it shed.

8Accept atonement, O Lord, for your people Israel, whom you have redeemed, and do not set the guilt of innocent blood in the midst of your people Israel, so that their blood guilt be atoned for.'

9So you shall purge the guilt of innocent blood from your midst, when you do what is right in the sight of the Lord.