Shabbos, Chapter 16, Mishnah 2. In the previous mishnah we learned that Chazal instituted a decree meant to prevent a person, in his panic at the time of a fire, from extinguishing the flames on Shabbos: they limited what may be taken out of a burning house, as long as there is no danger to life involved. The very restriction on what a person is permitted to remove reminds him that he may not put out the fire. The previous mishnah dealt with rescuing sacred writings and books, while our mishnah addresses the question of which food may be saved and in what quantity.
The text of the mishnah:
"Matzilin mezon shalosh se'udos" - one may rescue food in the amount of three meals. And the mishnah goes on to specify:
"HaRa'ui l'adam l'adam" - food fit for human consumption is rescued for people.
"V'hara'ui l'behemah l'behemah" - food suitable for an animal is rescued for the animal.
"Keitzad?" - the amount that may be rescued varies according to the time the fire breaks out:
"Naflah deleikah b'leil Shabbos - matzilin mezon shalosh se'udos" - if the fire broke out on Friday night before the meal, one rescues the equivalent of three meals, since he still has three meals to eat on Shabbos.
"B'shacharis - matzilin mezon shtei se'udos" - if the fire broke out in the morning before eating, one rescues only two meals and not three, since he has already eaten the first meal.
"B'minchah - mezon se'udah achas" - if the fire broke out at the time of Minchah, only the third meal remains ahead of him, and therefore one rescues only enough food for one meal.
The view of Rabbi Yosi:
"Rabbi Yosi omer: l'olam matzilin mezon shalosh se'udos" - Rabbi Yosi disagrees and holds that at any hour of Shabbos one rescues the amount of three meals. Three meals is the measure the Sages established, because that is the number of meals on Shabbos, and even if in his particular situation the person no longer needs three meals because he has already eaten one or two, he may always rescue the amount of three meals.
In summary: In this mishnah we learned the amount of food that may be rescued from a fire on Shabbos - food for three meals, with what is fit for a person given to a person and what is fit for an animal to an animal. According to the first Tanna the amount varies with the time of the fire: three meals on Friday night, two in the morning and one at Minchah; while according to Rabbi Yosi one always rescues the amount of three meals.