Exodus 32:27-29

27Then he said to them, “This is what the Lord, the God of Israel, says: ‘Each man strap a sword to his side. Go back and forth through the camp from one end to the other, each killing his brother and friend and neighbor.’ ” 28The Levites did as Moses commanded, and that day about three thousand of the people died. 29Then Moses said, “You have been set apart to the Lord today, for you were against your own sons and brothers, and he has blessed you this day.”