Exodus 34:19-20

19Every firstborn child that a woman gives birth to belongs to me. That is also true for every firstborn male that your animals give birth to. That includes your cows and your sheep.

20For a firstborn donkey, you may pay for it with a lamb instead. If you do not pay for it, you must break the donkey's neck. You must pay me back for every firstborn son that you have. Nobody may come to worship me without an offering to give me.