LXX Num 8:26

καὶ λειτουργήσει ὁ ἀδελφὸς αὐτοῦ ἐν τῆ σκηνῇ τοῦ μαρτυρίου φυλάσσειν φυλακάς, ἔργα δὲ οὐκ ἐργάζεται. οὕτως ποιήσεις τοῖς Λευείταις ἐν ταῖς φυλακαῖς αὐτῶν.

After that, they may assist their brothers in fulfilling their duties at the Tent of Meeting, but they themselves are not to do the work. This is how you are to assign responsibilities to the Levites.”