Exodus 32:27-28

He said to them, “Thus says the LORD God of Israel, ‘Every man strap 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 friend, and every man his neighbor [all who continue pagan worship].’ ” So the sons of Levi did as Moses instructed, and about three thousand men of the people [of Israel] were killed that day.