Exodus 32:28

26then Moses stood in the gate of the camp, and said, “Whoever is on the LORD’s side, come to me!” All the sons of Levi gathered themselves together to him.

27He said to them, “The LORD, the God of Israel, says, ‘Every man put his sword on his thigh, and go back and forth from gate to gate throughout the camp, and every man kill his brother, and every man his companion, and every man his neighbor.’”

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

29Moses said, “Consecrate yourselves today to the LORD, for every man was against his son and against his brother, that he may give you a blessing today.”

30On the next day, Moses said to the people, “You have sinned a great sin. Now I will go up to the LORD. Perhaps I shall make atonement for your sin.”