They had baked matzot cakes from the dough that they brought out of Egypt. It had no hametz, because they were thrust out of Egypt and could not delay, so they had not made provisions for themselves.
Now Adonai had said to Moses, “I will bring one more plague upon Pharaoh and on Egypt. After that, he will let you go from here. When he lets you go, he will surely thrust you out altogether from here.
Adonai said to Moses, “Now you will see what I am going to do to Pharaoh. By way of a strong hand he will let them go, and drive them out of his land.”
He told them another parable, “The kingdom of heaven is like hametz, which a woman took and hid in three measures of flour, until it was all leavened.”