Numbers 18:32
Once you have presented the best part of it, you will not incur guilt because of it. But you must not defile the sacred offerings of the Israelites, lest you die."

Numbers 19:2
"This is the statute of the law that the LORD has commanded: Instruct the Israelites to bring you an unblemished red heifer that has no defect and has never been placed under a yoke.