Reference for Pesachim 3:7
הַהוֹלֵךְ לִשְׁחֹט אֶת פִּסְחוֹ, וְלָמוּל אֶת בְּנוֹ, וְלֶאֱכֹל סְעֻדַּת אֵרוּסִין בְּבֵית חָמִיו, וְנִזְכַּר שֶׁיֶּשׁ לוֹ חָמֵץ בְּתוֹךְ בֵּיתוֹ, אִם יָכוֹל לַחֲזֹר וּלְבַעֵר וְלַחֲזֹר לְמִצְוָתוֹ, יַחֲזֹר וִיבַעֵר. וְאִם לָאו, מְבַטְּלוֹ בְלִבּוֹ. לְהַצִּיל מִן הַנָּכְרִים, וּמִן הַנָּהָר, וּמִן הַלִּסְטִים, וּמִן הַדְּלֵקָה, וּמִן הַמַּפֹּלֶת, יְבַטֵּל בְּלִבּוֹ. וְלִשְׁבֹּת שְׁבִיתַת הָרְשׁוּת, יַחֲזֹר מִיָּד:
If one were on his way [on the fourteenth of Nissan] to slaughter his Pesach offering, or to circumcise his son, or to eat the betrothal feast in the house of his father-in-law, and he remembered that he had chametz in his house — if he can return (home) and burn it and return to his mitzvah, he should do so; if not, he should void it in his heart.] (If he were on his way) to save someone [a Jew] from a troop [pursuing him], from a flood, from robbers, from a fire, or from a cave-in, he should nullify it in his heart. (If he were on his way) to establish resting for something permitted (as opposed to something that is a mitzvah) [i.e., If he were going to await night at the (Sabbath) bound, to establish "resting" there, so that he could walk two thousand cubits from there (on the festival) for the sake of something permitted], he must return immediately. [But if he were going to establish resting for the sake of a mitzvah, e.g., to go the next day to the house of a mourner or to a house of (mitzvah) feasting, it is as if he were going to slaughter his Pesach offering.]