Zevachim 2

The Gemara (2b) wonders regarding the source that סתמא לשמה, that sacrifices are valid when they are offered with no specific intent? It cannot be from the fact that our Mishna stated: Any sacrifice that was slaughtered not for their own sake, and it did not state: Any sacrifice that was not slaughtered for their own sake (which would mean that it was actively intended for another sake; however, if there was no intent at all, there would be no concern). It is noteworthy that in the Hagadah Shel Pesach we recite the following words רַבָּן גַּמְלִיאֵל הָיָה אוֹמֵר: כָּל שֶׁלֹּא אָמַר שְׁלשָׁה דְּבָרִים אֵלּוּ בַּפֶּסַח, לֹא יָצָא יְדֵי חוֹבָתוֹ, וְאֵלוּ הֵן:פֶּסַח, מַצָה וּמָרוֹר, Rabban Gamliel would say, “one who does not recite these three things on Pesach has not fulfilled his obligation, and they are: Pesach, Matzah and Maror.” Here too Rabban Gamliel used the expression כָּל שֶׁלֹּא אָמַר, one who does not say, as opposed to the words מי שאמר שלשה דברים אלו בפסח יצא ידי חובתו, one who did recite these three things has fulfilled his obligation. If Rabban Gamliel had used the positive tense then we could infer that perhaps one who inadvertently forgot to say these three things fulfills some sort of obligation. Now that Rabban Gamliel used the words that if one does not say these three things, he has not fulfilled his obligation, then there is no room for forgetfulness.