Numbers 19:9 (BSB)

Then a man who is ceremonially clean is to gather up the ashes of the heifer and store them in a ceremonially clean place outside the camp. They must be kept by the congregation of Israel for preparing the water of purification; this is for purification from sin.

From Numbers 19. Also in the ESV.

Commentary on Numbers 19:9

• Matthew Henry (Presbyterian), Matthew Henry's Complete Commentary on Numbers 19:1-10: We have here the divine appointment concerning the solemn burning of a red heifer to ashes, and the preserving of the ashes, that of them might be made, not a beautifying, but a purifying, water, for that was the utmost the law reached to; it offered not to adorn as the gospel does, but to cleanse only.
• John Gill (Reformed Baptist), Exposition of the Old and New Testaments on Numbers 19:9: And a man that is clean shall gather up the ashes of the heifer,.... A man, a clean priest, as the Targum of Jonathan; in later times great care was taken that the priest concerned in the burning of the red cow should be pure; he was separated from his own house seven days before the time, and every day he was sprinkled with the...
• Geneva Bible Notes (Reformed), Geneva Bible Study Notes on 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.
• Keil & Delitzsch (Lutheran), Biblical Commentary on the Old Testament on Numbers 19:2-10: Num 19:2-10 Preparation of the Purifying Water. - As water is the ordinary means by which all kinds of uncleanness are removed, it was also to be employed in the removal of the uncleanness of death. But as this uncleanness was the strongest of all religious defilements, fresh water alone was not sufficient to remove it; and consequently a certain kind of sprinkling-water was appointed...