Numbers 3:11-13

11Then Adonai said to Moses saying,

12“See, I have taken the Levites from among Bnei-Yisrael in place of the firstborn who open the womb among Bnei-Yisrael. The Levites will be Mine,

13because all the firstborn are Mine. In the day when I was striking down all the firstborn in the land of Egypt, I consecrated to Myself every firstborn in Israel—whether man or animal—as Mine. I am Adonai.”