Numbers 3:11, 12, 13

The Lord also said to Moses

“I am choosing the Levites from all the Israelites. They will take the place of all the firstborn children of Israel. The Levites will be mine.

All the firstborn are mine. When you were in Egypt, I killed all the firstborn children of the Egyptians. I took all the firstborn of Israel to be mine. Firstborn animals and children are mine. I am the Lord.”