Pesachim, Chapter 1, Mishnah 3. In this mishnah, Rabbi Yehudah and the Sages disagree about the times at which the search for chametz may be performed.
Rabbi Yehudah's view - three times for the search:
"Or l'arba'ah asar" - this is the primary time for the search: on the night of the fourteenth, as the day begins.
"U'v'arba'ah asar shacharis" - one who did not search on the night of the fourteenth searches the following morning.
"U'vish'as habi'ur" - one who did not search even in the morning searches at the time of destroying the chametz, at the beginning of the sixth hour, when chametz is forbidden by rabbinic law and one is obligated to destroy it.
The view of the Sages:
The Sages adopt a different measure, and at the first stage they agree with Rabbi Yehudah:
"Lo badak or arba'ah asar - yivdok b'arba'ah asar" - one who did not search on the night of the fourteenth as he should have, searches on the fourteenth, and they mean the entire day and not only the morning.
"Lo badak b'arba'ah asar - yivdok b'soch hamo'ed" - one who did not search on the fourteenth searches during the days of the festival.
"V'lo badak b'soch hamo'ed - yivdok l'achar hamo'ed" - even one who did not search during the festival itself is obligated to search after Pesach has ended, because of the concern of chametz that was owned over Pesach: chametz that was in his possession during Pesach became forbidden to derive benefit from, and therefore he must search for it even after the festival.
The root of the dispute:
The disagreement between Rabbi Yehudah and the Sages revolves around whether we are concerned that a person might come to eat the chametz he finds during his search. Rabbi Yehudah is concerned about this: once the hour has arrived when chametz is forbidden, one no longer searches for it, lest he find it and eat it inadvertently. The Sages hold that we are not concerned about this - it is not reasonable to assume that a person searching for chametz in order to destroy it will come to eat it in the middle of his search, and therefore the search continues even after the time of the prohibition and even after the festival has ended.
"U'mah sheme'shayer yanichenu b'tzin'ah":
The mishnah continues and deals with the chametz that remains in a person's hands: both the chametz he found in his search and the chametz he deliberately left for himself in order to eat it the next morning - for eating chametz is permitted on the morning of the fourteenth, even though the search was already performed on the night of the fourteenth, and therefore a person sets aside chametz for himself in addition to what he found. This chametz he must place in a concealed and guarded spot, "kedei shelo yehei tzarich bedikah acharav" - for if it were to be lost to him, he would be obligated to search again, since he does not know where that chametz is.
In summary: In this mishnah we learned the three times set by Rabbi Yehudah for the search for chametz - the night of the fourteenth, the morning of the fourteenth, and the hour of destruction; and the view of the Sages, that the search continues throughout the fourteenth, during the festival, and even afterward because of the prohibition of chametz that was owned over Pesach. The root of the dispute is the concern that one might come to eat the chametz while searching. Finally, we learned that the chametz that remains in one's possession must be placed discreetly and in a guarded spot, so that he not come to need an additional search.