Exodus 32:26-28

26So Moses stood at the edge of the camp. He said, ‘Everyone who decides to serve the Lord must come here to me.’ Then all the Levites went to stand near Moses.

27Then Moses said to the Levites, ‘The Lord, Israel's God, says this: Each one of you must take his sword in his hand. You must go everywhere in the camp, from this side to the other side. You must kill everyone, even your brothers, your friends and your neighbours.’

28The Levites obeyed Moses' command. About 3,000 Israelites died on that day.