Exodus 13:1-3

New Revised Standard Version, Anglicised Catholic Edition

13 The Lord said to Moses: ² Consecrate to me all the firstborn; whatever is the first to open the womb among the Israelites, of human beings and animals, is mine.

The Festival of Unleavened Bread

³ Moses said to the people, ‘Remember this day on which you came out of Egypt, out of the house of slavery, because the Lord brought you out from there by strength of hand; no leavened bread shall be eaten.