Numbers 3:11-13

Moses, I have chosen these Levites from all Israel, and they will belong to me in a special way. When I killed the first-born sons of the Egyptians, I decided that the first-born sons in every Israelite family and the first-born males of their flocks and herds would be mine. But now I accept these Levites in place of the first-born sons of the Israelites.