Numbers - Chapter 19

In this chapter we have the description of the sacrifice of the red heifer:

Bring a red heifer, a perfect animal that has no defects and has never been yoked to a plow. Give it to Eleazar the priest, and it will be taken outside the camp and slaughtered in his presence. Eleazar will take some of its blood on his finger and sprinkle it seven times toward the front of the Tabernacle. As Eleazar watches, the heifer must be burned—its hide, meat, blood, and dung. Eleazar the priest must then take a stick of cedar, a hyssop branch, and some scarlet yarn and throw them into the fire where the heifer is burning....  Then someone who is ceremonially clean will gather up the ashes of the heifer and deposit them in a purified place outside the camp. They will be kept there for the community of Israel to use in the water for the purification ceremony. This ceremony is performed for the removal of sin.

This is referred to in Hebrews, Chapter 9:

The blood of goats and bulls and the ashes of a heifer sprinkled on those who are ceremonially unclean sanctify them so that they are outwardly clean. How much more, then, will the blood of Christ, who through the eternal Spirit offered himself unblemished to God, cleanse our consciences from acts that lead to death, so that we may serve the living God!  For this reason Christ is the mediator of a new covenant, that those who are called may receive the promised eternal inheritance—now that he has died as a ransom to set them free from the sins committed under the first covenant.