Numbers 3:11-13

And the LORD spake unto Moses, saying,

I have taken the Levites ... The consecration of this tribe did not originate in the legislative wisdom of Moses, but in the special appointment of God, who chose them as substitutes for the firstborn. By an appointment made in memory of the last solemn judgment on Egypt, from which the Israelite households were miraculously exempt, all the firstborn were consecrated to God (Exodus 13:12; Exodus 22:29), who thus, under special circumstances, seemed to adopt the patriarchal usage of appointing the oldest to act as the priest of the family. But the privilege of redemption that was allowed the firstborn opened the way for a change: and accordingly, on the full organization of the Mosaic economy, the administration of sacred things formerly committed to the firstborn was transferred from them to the Levites, who received that honour partly as a tribute to Moses and Aaron, partly because this tribe had distinguished themselves by their zeal in the affair of the golden calf (Exodus 32:29), and also because, being the smallest of the tribes, they could ill find suitable employment and support in the work (see the note at Deuteronomy 33:9).

The designation of a special class for the sacred offices of religion was a wise arrangement; because, on their settlement in Canaan, the people would be so occupied that they might not be at leisure to wait on the service of the sanctuary, and sacred things might, from various causes, fall into neglect. But the appointment of an entire tribe to the divine service ensured the regular performance of the rites of religion. The subsequent portion of the chapter relates the formal substitution of this tribe.

I am the Lord - i:e., I decree it to be so; and being possessed of sovereign authority, expect full obedience.