Rashi on Numbers 8:17/רש"י על במדבר ח׳:י״ז

1כי לי כל בכור FOR ALL THE FIRSTBORN [OF THE CHILDREN OF ISRAEL] ARE MINE — This means: they became Mine by strict right (lit., by the line of justice), because I protected them distinguishing between them and the firstborn of Egypt, and I took them as Mine — until they sinned by worshipping the Golden Calf, but now ואקח את הלוים I HAVE TAKEN THE LEVITES in their stead.