Numbers 19:9, 17

“A man who is clean is to gather up the cow’s ashes and deposit them outside the camp in a ceremonially clean place. The ashes will be kept by the Israelite community for preparing the water to remove impurity; it is a sin offering.

For the purification of the unclean person, they are to take some of the ashes of the burnt sin offering, put them in a jar, and add fresh water to them.