Deuteronomy 16:3

Thou shalt eat no leavened bread with it
With the passover, as the Targum of Jonathan expresses it; that is, with the passover lamb, nor indeed with any of the passover, or peace offerings, as follows; see ( Exodus 12:8 )

seven days shall ye eat unleavened bread therewith;
with the passover; this plainly shows, that by the passover in the preceding verse is not meant strictly the passover lamb, for that was eaten at once on the night of the fourteenth of the month, and not seven days running, and therefore must be put for the whole solemnity of the feast, and all the sacrifices of it, both the lamb of the fourteenth, and the Chagigah of the fifteenth, and every of the peace offerings of the rest of the days were to be eaten with unleavened bread:

[even] the bread of affliction;
so called either from the nature of its being heavy and lumpish, not grateful to the taste nor easy of digestion, and was mortifying and afflicting to be obliged to eat of it seven days together; or rather from the use of it, which was, as Jarchi observes, to bring to remembrance the affliction they were afflicted with in Egypt:

for thou camest forth out of the land of Egypt in haste;
and had not time to leaven their dough; so that at first they were obliged through necessity to eat unleavened bread, and afterwards by the command of God in remembrance of it; see ( Exodus 12:33 Exodus 12:34 Exodus 12:39 ) ,

that thou mayest remember the day when thou camest forth out of the
land of Egypt all the days of thy life;
how it was with them then, how they were hurried out with their unleavened dough; and that this might be imprinted on their minds, the master of the family used ^F16, at the time of the passover, to break a cake of unleavened bread, and say, this is the bread of affliction or bread of poverty; as it is the way of poor men to have broken bread, so here is broken bread.