In Pesachim (16a), it is stated: "Rabban Gamliel would say: Anyone who has not mentioned these three things on Pesach has not fulfilled his obligation: Pesach, matzah, and maror. Pesach - because our ancestors would eat it when the Temple stood, as it is stated (Exodus 12:27), 'You shall say: It is a Pesach-offering to the Lord.' Matzah - because our ancestors were redeemed from Egypt, as it is stated (ibid 39), 'They baked the dough that they brought out of Egypt.' Maror - because the Egyptians embittered the lives of our ancestors in Egypt, as it is stated (ibid 1:14), 'And they embittered their lives'." The Tosafot commentary on Pesachim (there, s.v. ve'amartem) explains: "'You shall say: It is a Pesach-offering' - meaning, in the statement, that one must specify that this is the Pesach that we eat, and there is a comparison drawn between matzah and maror to Pesach, and one must also say 'this matzah', 'this maror'."