Tevul Yom, Chapter 3, Mishnah 6. The theme running through this chapter is the question of chibbur, connection: when a tevul yom touches one point of a food, do we regard the food as a single connected unit, so that the entire item becomes possul, or do we treat it as separate pieces, so that only the exact spot he touched is affected? Our mishnah applies that question to a cooked dish.
The scenario: "Yarak shel chullin shebishlo beshemen shel terumah" - a vegetable belonging to chullin that was cooked in oil of terumah. The produce is plain, unconsecrated food; whatever terumah standing the dish has comes entirely from the oil, which has soaked into the vegetable and clings to its outside. Now, "venaga bo tevul yom": someone who immersed today and is still waiting for sunset lays his hand on that dish at one particular spot.
Two traditions are recorded about this case. The first is transmitted by Rabbi Elazar ben Yehuda of Bartosa, who reports what he received "mishum Rabbi Yehoshua" - a ruling he heard from Rabbi Yehoshua: "passal es kulah," the dish has been invalidated in its entirety. The logic is that the oil together with the vegetable forms one bound-together mass, so contact with any part of it counts as contact with the whole.
"Rabbi Akiva omer mishmo: lo passal ela mekom maga'o." Rabbi Akiva cites that same Rabbi Yehoshua in the opposite direction: only the place of contact became possul. In his understanding, this is not treated as a single unified dish but as a quantity of individual vegetables lying alongside one another, and therefore the tevul yom has disqualified nothing beyond the spot his hand actually reached.
So the mishnah preserves two versions of Rabbi Yehoshua's ruling on a chullin vegetable cooked in terumah oil, and the entire dispute turns on a single point: whether the oil binds the dish into one item, or whether the pieces remain halachically separate from one another.