Exodus 34:18-20

Young's Literal Translation

¹⁸ `The feast of unleavened things thou dost keep; seven days thou dost eat unleavened things, as I have commanded thee, at an appointed time, the month of Abib: for in the month of Abib thou didst come out from Egypt.

¹⁹ `All opening a womb [are] Mine, and every firstling of thy cattle born a male, ox or sheep;

²⁰ and the firstling of an ass thou dost ransom with a lamb; and if thou dost not ransom, then thou hast beheaded it; every first-born of thy sons thou dost ransom, and they do not appear before Me empty.