Numbers 3:12

All male first-born of men and animals are sacred to God, i.e. they must be sacrificed. In very primitive ages it is probable that this custom actually prevailed (cf. Genesis 22; 2 Kings 3:27; Micah 6:7), but the Israelite practice of redeeming human first-born by an equivalent in money or animals must have arisen at an early date. The practice of offering the firstborn is enjoined in J E (Exodus 22:29 (E), Numbers 13:11-33 [Note: J, however, differs from P in dating the offering of first-born not from the Exodus but from the entrance into Canaan.], Numbers 34:19 f. (J)), Numbers 18:15 f. (P), and the redemption (in P) is to be by a money payment. But at this point, in P, a different form of redemption is commanded, i.e. the handing over of the Levites for service. This subject is resumed in Numbers 3:40, where it is explained that the Levites were substituted only for those firstborn who were more than a month old at the time.