Bilangan 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.

Bilangan 29:10

29:10 and one-tenth for each of the seven lambs,

[19:12]  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.