Exodus 32:26-29

26then Moses stood in the gate of the camp and said, "Who is on the Lord's side? Come to me." And all the sons of Levi gathered around him.

27And he said to them, "Thus says the Lord God of Israel, 'Put your sword on your side each of you, and go to and fro from gate to gate throughout the camp, and each of you kill his brother and his companion and his neighbor.'"

28And the sons of Levi did according to the word of Moses. And that day about three thousand men of the people fell.

29And Moses said, "Today you have been ordained for the service of the Lord, each one at the cost of his son and of his brother, so that he might bestow a blessing upon you this day."