Numbers 18:26

18:26 “You are to speak to the Levites, and you must tell them, ‘When you receive from the Israelites the tithe that I have given you from them as your inheritance, then you are to offer up ¹  from it as a raised offering to the Lord a tenth of the tithe.

Exodus 29:28

29:28 It is to belong to Aaron and to his sons from the Israelites, by a perpetual ordinance, for it is a contribution. It is to be a contribution from the Israelites from their peace offerings, their contribution to the Lord.

[18:26]  ¹ tn The verb in this clause is the Hiphil perfect with a vav (ו) consecutive; it has the same force as an imperfect of instruction: “when…then you are to offer up.”