Exodus 12:39

And they baked unleavened cakes of the dough which they brought forth out of Egypt, for it was not leavened; these unleavened cakes were the only provision they had, for their deliverance came upon them much more quickly than they had looked for; because they were thrust out of Egypt, and could not tarry, neither had they prepared for themselves any victual. Thus they celebrated, for the first time, the Feast of Unleavened Bread. And so the name of the Lord was magnified by this great deliverance, which remained a source of inspiration to the Hebrew poets for many hundreds of years, even as we Christians sing the praises of the eternal redemption which was gained for us by Christ.