John Gill Commentary Numbers 31:21

"And Eleazar the priest said unto the men of war that went to the battle, This is the statute of the law which Jehovah hath commanded Moses:" — Numbers 31:21 (ASV)

And Eleazar the priest said to the men of war, who went to
the battle
To all the soldiers, officers, and common men:

this is the ordinance of the law, which the Lord commanded Moses ;
concerning the purification of unclean persons and things, hereby confirming what Moses had said.