Exodus 12:15, 39

For seven days the bread you eat must be made without yeast. On the first day of the festival, remove every trace of yeast from your homes. Anyone who eats bread made with yeast during the seven days of the festival will be cut off from the community of Israel.

For bread they baked flat cakes from the dough without yeast they had brought from Egypt. It was made without yeast because the people were driven out of Egypt in such a hurry that they had no time to prepare the bread or other food.