Midrash su Levitico 23:6

2) (Vayikra 23:6) ("And on the fifteenth day of this month is the festival of matzoth to the L–rd. Seven days shall you eat matzoh.") "And on the fifteenth day of this month is the festival of matzoth": This day requires matzoh, but the festival of Succoth does not require matzoh. For would it not follow otherwise, a fortiori, viz.: If this (Pesach), which does not require a succah (a booth) requires matzoh, then that (Succoth), which does require a succah, how much more so should it require matzoh! It is, therefore, written "this" ("of this month). It is this festival of matzoth, which requires matzoh, but not the festival of Succoth.

3) "the festival of matzoth to the L–rd, seven days, etc." What is the intent of this? Because it is written (Devarim 16:8) "Six days shall you eat matzoth and on the seventh day it is a cessation (atzereth) for the L–rd" — The seventh day was included in the general (mitzvah of matzoh, viz.: [Shemoth 13:6] "Seven days shall you eat matzoth; and it left that general category to serve as a paradigm, viz.: Just as (the eating of matzoth on) the seventh day is permitted (and not obligatory), so, all (of the days) are permitted (in this regard, and not obligatory.) I might think that (the eating of matzoth on) the first night, too, is permitted (and not obligatory); it is, therefore, written (Devarim 16:3) "Upon it shall you eat matzoth" — Scripture makes it mandatory.

4) This tells me only of the time that the Temple existed. Whence do I derive the same for these days, when the Temple does not exist (and there is no Paschal lamb)? From (Shemoth 12:18) "In the evening you shall eat matzoth."