Numbers 25:1-3, 5

And Israel abode in Shittim, and the people began to commit whoredom with the daughters of Moab. And they called the people unto the sacrifices of their gods: and the people did eat, and bowed down to their gods. And Israel joined himself unto Baal-peor: and the anger of the LORD was kindled against Israel.

And Moses said unto the judges of Israel, Slay ye every one his men that were joined unto Baal-peor.