Reference for Sukkah 2:5
מַעֲשֶׂה וְהֵבִיאוּ לוֹ לְרַבָּן יוֹחָנָן בֶּן זַכַּאי לִטְעוֹם אֶת הַתַּבְשִׁיל, וּלְרַבָּן גַּמְלִיאֵל שְׁתֵּי כוֹתָבוֹת וּדְלִי שֶׁל מַיִם, וְאָמְרוּ, הַעֲלוּם לַסֻּכָּה. וּכְשֶׁנָּתְנוּ לוֹ לְרַבִּי צָדוֹק אֹכֶל פָּחוֹת מִכַּבֵּיצָה, נְטָלוֹ בַמַּפָּה וַאֲכָלוֹ חוּץ לַסֻּכָּה, וְלֹא בֵרַךְ אַחֲרָיו:
Once they brought R. Yochanan b. Zakkai a dish to taste and (they brought) to R. Gamliel two dates and a waterskin, and they said: "Take them up to the succah." [Not because it was required, but they were stringent with themselves — whence it is derived that if one is stringent with himself, not to eat even "casually" outside the succah, he is to be commended.] And when they gave to R. Tzaddok food less than the size of an egg, [so that he not have to recite the (concluding) blessing [(For, as far as succah is concerned, it has already been stated that one may eat "casually" outside the succah — even more than the size of an egg)], he took it in a cloth, ate it outside the succah, and did not recite the concluding blessing.