Exodus 13:11-13

11The Lord will bring you into the land of the Canaanites. It is the land that he promised to give to you and to your ancestors. When he gives it to you, you must do this:

12You must give to the Lord all the firstborn males. That includes your own firstborn sons, and the male animals that are born first. They belong to the Lord.

13When a donkey gives birth to her first male, you must give a lamb to God, instead of the donkey. If you do not do that, you must kill the donkey. You must break its neck. For any of your own firstborn sons, you must give an animal instead of your son.