Numbers 19:9

A man that is clean. That is, free from ceremonial impurity. He must be another person than the man who burned the heifer.

Gather up the ashes. That is, of the heifer, the cedar, the hyssop, and the scarlet thread, blended together.

Lay them up. For mixing with running water, as needed, to supply water for the cleansing of ceremonial impurity (v. 17). Compare this with the ashes of the golden calf (Ex. 32:20).

It shall be kept. That is, the ashes. The Hebrew word is singular in number.

For a water of separation. That is, for the purification of men who, having become unclean, were separated or banished, as it were, from the congregation. The Hebrew word translated “separation” signifies anything loathsome, such as idolatry or immorality. It is also rendered “filthiness” (2 Chron. 29:5) and “uncleanness” (Ezra 9:11; Zech. 13:1).