Charles Ellicott Commentary Numbers 19:17

"And for the unclean they shall take of the ashes of the burning of the sin-offering; and running water shall be put thereto in a vessel:" — Numbers 19:17 (ASV)

of the ashes of the burnt heifer of purification for sin. —Better, of the ashes of the burnt sin-offering; literally, of the burning of the sin-offering.