Mishnayos Pesachim Perek 10 Mishnah 5

Rabban Gamliel would say: Anyone who did not say these three matters on Passover has not fulfilled his obligation: The Paschal lamb, matza, and bitter herbs. When one mentions these matters, he must elaborate and explain them: The Paschal lamb is brought because the Omnipresent passed over [pasaḥ] the houses of our forefathers in Egypt, as it is stated: “That you shall say: It is the sacrifice of the Lord’s Paschal offering for He passed over the houses of the children of Israel in Egypt, when he smote the Egyptians, and delivered our houses” (Exodus 12:27). Rabban Gamliel continues to explain: The reason for matza is because our forefathers were redeemed from Egypt, as it is stated: “And they baked the dough that they took out of Egypt as cakes of matzot, for it was not leavened, as they were thrust out of Egypt and could not tarry, neither had they prepared for themselves any victual” (Exodus 12:39). The reason for bitter herbs is because the Egyptians embittered our forefathers’ lives in Egypt, as it is stated: “And they embittered their lives with hard service, in mortar and in brick; in all manner of service in the field, all the service that they made them serve was with rigor” (Exodus 1:14). The tanna of the mishna further states: In each and every generation a person must view himself as though he personally left Egypt, as it is stated: “And you shall tell your son on that day, saying: It is because of this which the Lord did for me when I came forth out of Egypt” (Exodus 13:8). In every generation, each person must say: “This which the Lord did for me,” and not: This which the Lord did for my forefathers. The mishna continues with the text of the Haggadah. Therefore we are obligated to thank, praise, glorify, extol, exalt, honor, bless, revere, and laud [lekales] the One who performed for our forefathers and for us all these miracles: He took us out from slavery to freedom, from sorrow to joy, from mourning to a Festival, from darkness to a great light, and from enslavement to redemption. And we will say before Him: Halleluya. At this point one recites the hallel that is said on all joyous days. Since one does not complete hallel at this point in the seder, the mishna asks: