Exodus 32:26-28 NIV

26 So he stood at the entrance to the camp and said, "Whoever is for the LORD, come to me." And all the Levites rallied to him.
27 Then 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.' "1

References for Exodus 32:27

28 The Levites did as Moses commanded, and that day about three thousand of the people died.