LXX Num 8:16

ὃτι απόδομα ἀποδεδομένοι οὑτοιὶ μοί εἰσιν ἐκ μέσου υἱῶν Ἰσραήλ· ἀντὶ τῶν διανοιγόντων πᾶσαν μήτραν πρωτοτόκων πάντων τῶν ἐκ τῶν υἱῶν Ἰσραὴλ εἴληφα αὐτοὺς ἐμοί.

For the Levites have been wholly given to Me from among the sons of Israel. I have taken them for Myself in place of all who come first from the womb, the firstborn of all the sons of Israel.