Pesachim Chapter 1, Mishnah 4. This mishnah focuses on the question of exactly when a person must stop eating chametz, and when he must burn his chametz.
Rabbi Meir says:
"Ochlin kol chamesh" - one may eat chametz throughout the entire fifth hour of the day.
"Vesorfin bitchilas shesh" - the burning of the chametz must begin immediately at the start of the sixth hour.
The hours mentioned in the mishnah are halachic hours (proportional hours): the time between sunrise and sunset is divided into twelve equal parts, and each such part is called an hour. Thus, on a day that begins at six in the morning and ends at six in the evening, one may eat chametz until eleven in the morning, and as soon as the fifth hour ends - at the start of the sixth hour - one must begin burning the chametz.
It emerges that the two times follow one directly after the other, and the burning takes place during the sixth hour. And even though the Torah prohibition against eating chametz takes effect only after six full hours of the day, the Sages wished to distance a person from transgression, and therefore they added one hour and instructed that the burning of the chametz begin already with the onset of the sixth hour.
Rabbi Yehudah says:
"Ochlin kol arba" - one may eat chametz only until the end of the fourth hour of the day, and in our example: until ten in the morning.
"Vetolin kol chamesh" - throughout the entire fifth hour, from ten until eleven, the matter is suspended and in doubt.
"Vesorfin bitchilas shesh" - the burning takes place at the start of the sixth hour, as Rabbi Meir said.
Rabbi Yehudah's concern is that since determining the time is left to each individual, a person is liable to err even by as much as two hours. True, it is generally difficult to confuse the fifth hour with the sixth, since during the sixth hour the sun stands at its highest point, while during the fifth hour it has already moved slightly away from there, and an ordinary person does not err in this; but at times the day is cloudy and it is impossible to discern this, and therefore they said to move the time up by two hours.
Nevertheless, during the fifth hour there is no obligation to burn, because the chametz at that time - although forbidden to eat - is still permitted to benefit from even according to Rabbi Yehudah. At that time one may sell it to a non-Jew and the like, and only eating it has been forbidden.
In summary: in this mishnah the Tannaim disagree about two times: the time for stopping the eating of chametz and the time for burning it. According to Rabbi Meir, one eats throughout the fifth hour and burns at the start of the sixth; according to Rabbi Yehudah, one eats until the end of the fourth, during the fifth the matter is suspended, and at the start of the sixth one burns - all out of concern for error in determining the hours, and based on the distinction between the prohibition of eating and the prohibition of benefit.