Exodus 34:18-20

18The feast of unleavened bread shalt thou keep. Seven days shalt thou eat unleavened bread, as I commanded thee, in the time of the month Abib: for in the month Abib thou camest out of from Egypt. 19All that openeth the womb is mine; and every firstling among thy cattle, whether ox or sheep, that is male .1 20But the firstling of a donkey thou shalt redeem with a lamb: and if thou shalt not redeem him, then shalt thou break his neck. All the firstborn of thy sons thou shalt redeem. And none shall appear before me empty.2