Pesachim, Chapter Ten, Mishnah 5. We continue describing the details of what is done on the Seder night, and in fact this Mishnah is taken directly from the Haggadah, or more precisely, the Haggadah is taken from it.
Rabban Gamliel Used to Say:
The Mishnah states: "Kol shelo amar sheloshah devarim eilu baPesach, lo yatza yedei chovaso" - whoever has not said these three things on Pesach has not fulfilled his obligation. In other words, one who has not explained these three matters has not fulfilled the obligation of telling the story of the exodus from Egypt. And they are: the Pesach offering, the eating of matzah, and the maror.
"Pesach" - because the Omnipresent passed over the houses of our fathers in Egypt, that the Holy One, blessed is He, skipped over the houses of our fathers.
"Matzah" - because our fathers were redeemed from Egypt. In our Haggadah a fuller explanation is brought: because the dough of our fathers did not have time to become leavened.
"Maror" - because the Egyptians embittered the lives of our fathers in Egypt.
In Every Generation:
The Mishnah continues with the words we go on to say in the Haggadah: "B'chol dor vador chayav adam liros es atzmo k'ilu hu yatza mimitzrayim" - in every generation a person is obligated to see himself as though he personally went out of Egypt. This principle is learned from the verse, and as Rashi explains in the Chumash, that we are commanded to tell our son on that day, in time to come, saying: "Ba'avur zeh asah Hashem li b'tzeisi mimitzrayim" - for the sake of this Hashem acted for me when I went out of Egypt, meaning for the sake of my keeping His mitzvos.
The Obligation to Praise:
Therefore we continue and say that we are obligated in every form of praise to the One who did all these miracles for our fathers and for us. He took us out:
From slavery to freedom.
From sorrow to joy.
From mourning to a festival.
From darkness to a great light.
From subjugation to redemption.
"V'nomar lefanav" - and let us say before Him: we praise His Name. At this point we move on to the beginning of Hallel.