Pesachim, Chapter 3, Mishnah 7. This mishnah addresses a case where the obligation to destroy chametz conflicts with other mitzvos that a person is obligated in at that very moment, and the question of which one gives way to the other.
The text of the mishnah:
"HaHolech lishchot es pischo" - one who left his home in order to slaughter and bring the Pesach offering.
"Velamul es beno" - or who went out to perform the bris milah of his son.
"Ve'le'echol se'udas eirusin b'veis chamiv" - or who went out to an engagement meal in the home of his future father-in-law.
While he is on the way to fulfill these mitzvos, "venizkar sheyesh lo chametz b'toch beiso" - he remembers that he has chametz in his house. True, every person has the ability to nullify his chametz, but even one who nullifies it is obligated to actually destroy it. The question, then, is whether in such a situation he may rely on nullification alone, or whether he must return home and actually destroy the chametz.
On this point the mishnah distinguishes between two situations:
"Im yachol lachzor u'le'va'er velachzor lemitzvaso - yachzor viva'er" - when there is enough time to go back, destroy the chametz, and afterward fulfill the mitzvah, he is obligated to return and destroy it first, and only then fulfill his mitzvah.
"Ve'im lav - mevatel b'libo" - when there is not enough time for both, he fulfills the mitzvah and suffices with nullifying the chametz. The formula of nullification is said aloud, and he is not required to actually destroy the chametz.
Mitzvos involving saving lives:
The law is different when a person sets out for a mitzvah that involves saving lives, which is a far greater mitzvah, and the mishnah lists several such cases:
"Lehatzil min hagayis" - to save people from an invading army.
"Min hanahar" - from a flooding river.
"U'min halistim" - from the hands of bandits.
"U'min hadelek" - from a fire.
"U'min hamapoles" - from a collapsed building, such as in an earthquake.
In all of these "yevatel b'libo" - and here no distinction is made between one who has time to return and one who does not, since in all these cases time is critical, and the sooner he arrives the better.
Optional rest:
In contrast, "Velishbos shevisas harshus - yachzor miyad" - one who was on his way to place an eiruv techumin for an optional purpose, something that involves no significant mitzvah but is merely something he wanted to do, and he remembered that he has chametz at home, must return immediately and destroy the chametz.
However, if the purpose for which he placed the eiruv techumin was for a mitzvah, such as comforting mourners, gladdening a groom, going to the beis midrash, greeting his teacher and the like, his law is the same as that of one going to slaughter the Pesach offering, even though it is not as great a mitzvah as that one: if there is time, he goes and destroys the chametz and returns to his mitzvah, and if not, he suffices with nullification alone.
The question of Tosafos:
Tosafos raise a question here: the halachah is that one may only make an eiruv techumin for the purpose of a mitzvah. If so, seemingly all the cases fall under the category of a mitzvah, so what is this "optional rest" for which he must return immediately and destroy the chametz?
Tosafos answer that the case is one who is going merely to enjoy himself, to take pleasure and rejoice on the festival of Pesach in the home of his friend or relative. There is enough of a mitzvah in this to permit placing an eiruv techumin, but it is not a mitzvah for which the destruction of chametz is set aside - destroying the chametz takes precedence over such a minimal mitzvah.
In summary: In this mishnah we learned the order of priorities when destroying chametz conflicts with other mitzvos: one who set out to slaughter his Pesach offering, to circumcise his son, or to attend an engagement meal, if there is time he returns and destroys the chametz, and if not he nullifies it in his heart; one who set out to save lives nullifies it in his heart in any case because of the urgency of the moment; and one who goes to establish an optional rest returns immediately and destroys the chametz, while a rest for the purpose of a mitzvah follows the law of one going to slaughter his Pesach offering. And according to Tosafos, even "optional rest" contains a trace of a mitzvah that permits an eiruv techumin, but it does not have the power to set aside the destruction of chametz.