Bavli Pesahim 119b-120a

(1a) R. Judah said in the name of Shmuel: One may not conclude with an afikoman after the matzah.

(2a) We learned [in a mishnah]: one may not conclude after the pesah with an afikoman.

After the pesah is when this is prohibited but after the matzah you may have an afikoman.

(3a) It wasnt even necessary to state: It wasnt even necessary to state that after the matzah [there is no afikoman], since its taste is not substantial; after the pesah, whose taste is substantial and cannot [easily] be wiped out [I might have thought] that it would not be prohibited [to have an afikoman], hence [the mishnah] teaches us [that it is prohibited].

(4a) Shall we say that the following supports him: [As for] sponge cakes, honey-cakes and iskeritin, a man may fill his stomach with them, providing that he eat an olives worth of matzah at the end. [This implies], only at the end, but not at the beginning!

(5a) It wasnt even necessary to state: It wasnt even necessary to state that [if he eats it] at the beginning [that he has fulfilled his obligation], since he eats it with an appetite; but at the end, lest he may come to eat it as mere gorging, I might say [that he does not thereby fulfill his obligation] hence it teaches us [that he does].

(1b) Mar Zutra taught it thus: R. Yosef said in Rav Judah’s name in Samuel’s name: One may conclude after the matzah with an afikoman.

(2b) Shall we say that the following mishnah supports him: one may not conclude after the pesah with an afikoman.

After the pesah is when this is prohibited but after the matzah you may have an afikoman.

(3b) It wasnt even necessary to state: It wasnt even necessary to state that after the matzah [there is no afikoman], since its taste is not substantial; after the pesah, whose taste is substantial and cannot [easily] be wiped out [I might have thought] that it would not be prohibited [to have an afikoman], hence [the mishnah] teaches us [that it is prohibited].

(4b) They objected: [As for] sponge cakes, honey-cakes and iskeritin, a man may fill his stomach with them, providing that he eat an olives worth of matzah at the end. [This implies], only at the end, but not at the beginning!

(5a) It wasnt even necessary to state: It wasnt even necessary to state that [if he eats it] at the beginning [that he has fulfilled his obligation], since he eats it with an appetite; but at the end, lest he may come to eat it as mere gorging, I might say [that he does not thereby fulfill his obligation] hence it teaches us [that he does].