Exodus 34:18-20

18The feast of unleavened bread shalt thou keep. Seven days thou shalt eat unleavened bread, as I commanded thee, at the time appointed in the month Abib: for in the month Abib thou camest out from Egypt.

19All that openeth the womb is mine; and all thy cattle that is male, the firstlings of ox and sheep.

20And the firstling of an ass thou shalt redeem with a lamb: and if thou wilt not redeem it, then thou shalt break its neck. All the firstborn of thy sons thou shalt redeem. And none shall appear before me empty.