Sukkah, Chapter Two, Mishnah 5. Our Mishnah addresses the question: what quantity of food obligates a person to eat in the sukkah.
The text of the Mishnah:
"Ma'aseh v'hevi'u lo l'Rabban Yochanan ben Zakkai lit'om et hatavshil, ul'Rabban Gamliel shtei temarim u'dli shel mayim, v'amru: Ha'alum la'sukkah" - Rabban Yochanan ben Zakkai was brought a small amount of cooked food, just enough for a taste and not a real meal; Rabban Gamliel was brought two dates and a small bucket of water. In both cases they instructed: bring them up to the sukkah, and we will eat them there.
In neither of these cases was the quantity a mere casual snack; rather, it was eating of a more formal character, the kind that carries an obligation of sukkah. Even so, we learn from here that there is value in being stringent, whenever possible, and eating in the sukkah even the smallest amount - even a mere taste. This is not an obligation, but it is a praiseworthy practice, and we find great men of Israel who conducted themselves this way.
The incident with Rabbi Tzadok - the opposite example:
"V'shenatnu lo l'Rabbi Tzadok ochel pachot mi'kabeitzah, netalo b'mappah v'achalo chutz la'sukkah" - Rabbi Tzadok was given food that was less than the volume of an egg, that is, less than the amount that obligates one in sukkah. He took it with a napkin and did not touch it with his hands, because he was about to eat it without washing his hands. Under what circumstances a person may act this way is a complex matter in its own right, but in this case it was permitted to him, and therefore he did not touch the food directly.
And that same food he ate outside the sukkah, since as a matter of practical halachah, less than the volume of an egg carries no obligation of sukkah. Rabbi Tzadok, for whatever reason, did not want to or was not able to be stringent and follow that praiseworthy practice, but rather kept to the basic halachah alone, and therefore ate outside the sukkah.
"V'lo beirach acharav" - he also did not recite a berachah after eating, because he held like Rabbi Yehudah (and this is a dispute in Tractate Berachos), that a person is not obligated in Birkas HaMazon unless he has eaten the volume of an egg. It is worth emphasizing that this ruling applies only to the berachah after eating; the first berachah, before eating, applies to a person with any quantity at all, whatever the amount of food may be.
In summary: in this Mishnah we saw two complementary incidents - Rabban Yochanan ben Zakkai and Rabban Gamliel, who brought up to the sukkah even a small amount of cooked food, dates and water, and from them we learn the praiseworthy practice of eating in the sukkah even a mere taste; and in contrast, Rabbi Tzadok, who ate less than the volume of an egg outside the sukkah in accordance with the basic law, and also did not recite a berachah afterward, following the view that there is no obligation of a berachah after eating with less than the volume of an egg.