Exodus 13:11-13

11“This is what you must do when the Lord fulfills the promise he swore to you and to your ancestors. When he gives you the land where the Canaanites now live,

12you must present all firstborn sons and firstborn male animals to the Lord, for they belong to him.

13A firstborn donkey may be bought back from the Lord by presenting a lamb or young goat in its place. But if you do not buy it back, you must break its neck. However, you must buy back every firstborn son.