Numbers 19:9
And a man that is clean shall gather up the ashes of the heifer, and lay them up without the camp in a clean place, and it shall be kept for the congregation of the children of Israel for a water of separation: it is a purification for sin.
Cross-reference
Numbers 19:13 warns of the consequence for failing to use the water of separation made from these ashes.
Numbers 19:17 instructs using the ashes with running water to create the water of separation.
Numbers 19:18 describes using the water of purification made from the ashes of verse 9 to sprinkle on the unclean.
Numbers 19:20 repeats the warning for those who neglect purification with the water of separation.
Numbers 31:23 applies the water of separation (from these ashes) to purify spoils of war that endure fire.
Numbers 8:7 uses the same 'water of purification' from the red heifer to cleanse Levites, linking to the ashes ritual.
Zechariah 13:1 promises a fountain to cleanse sin and uncleanness, echoing the water of purification from the red heifer ashes.
Hebrews 9:13 cites the ashes of a heifer as an example of old covenant purification that sanctifies the flesh.
Hebrews 9:14 contrasts the limited purification of ashes with Christ's blood that cleanses the conscience.