Num Numbers, chapter Book of Numbers, chapter 3
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; Exo 29:24; Num 8:16Because 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.Exo 13:2; Exo 22:28; Exo 34:19; Lev 27:26; Num 8:17; Luk 2:23