Exodus 12:6, 18-20

and ye shall keep it until the fourteenth day of the same month; and the whole assembly of the congregation of Israel shall kill it at even.

In the first month, on the fourteenth day of the month at even, ye shall eat unleavened bread, until the one and twentieth day of the month at even. Seven days shall there be no leaven found in your houses: for whosoever eateth that which is leavened, that soul shall be cut off from the congregation of Israel, whether he be a sojourner, or one that is born in the land. Ye shall eat nothing leavened; in all your habitations shall ye eat unleavened bread.