Exodus 34:18-20

18“You must celebrate the Festival of Unleavened Bread. For seven days the bread you eat must be made without yeast, just as I commanded you. Celebrate this festival annually at the appointed time in early spring, in the month of Abib, for that is the anniversary of your departure from Egypt.

19“The firstborn of every animal belongs to me, including the firstborn males from your herds of cattle and your flocks of sheep and goats.

20A 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. “No one may appear before me without an offering.