Bamidbar 3:12-13

And I, hinei, I have taken the Levi'im from among the Bnei Yisroel instead of all the bechor that openeth the womb among the Bnei Yisroel; therefore the Levi'im shall be Mine; Because all the bechor are Mine; for on the yom that I struck down all the bechor in Eretz Mitzrayim I set apart as kodesh unto Me all the bechor in Yisroel, both man and beast: Mine shall they be: I am HASHEM.