Numbers 19:12

19:12 He must purify himself ¹  with water on the third day and on the seventh day, and so will be clean. But if he does not purify himself on the third day and the seventh day, then he will not be clean.

Numbers 31:19

Purification After Battle

31:19 “Any of you who has killed anyone or touched any of the dead, remain outside the camp for seven days; purify yourselves and your captives on the third day, and on the seventh day.

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¹ tn The verb is the Hitpael of חָטָא (khata’), a verb that normally means “to sin.” But the Piel idea in many places is “to cleanse; to purify.” This may be explained as a privative use (“to un-sin” someone, meaning cleanse) or denominative (“make a sin offering for someone”). It is surely connected to the purification offering, and so a sense of purify is what is wanted here.