Mishnayos Pesachim Perek 3 Mishnah 7

One who is traveling on the eve of Passover to slaughter his Paschal lamb, to circumcise his son, or to eat a betrothal feast in his father-in-law’s house, and he remembers that he has leavened bread in his house, if he is able to return to his house and remove the leaven and afterward return to the mitzva toward which he was traveling, he should return home and remove his leaven. But if there is not enough time for him to go home and remove the leaven, and still complete the mitzva that he already began, he should nullify it in his heart, as by Torah law this is sufficient. If one was traveling to save Jews from an attack by gentiles, from a flooding river, from bandits, from a fire, or from a collapsed building, he should not even attempt to return, and instead he should nullify the leaven in his heart. This applies even if he could remove his leaven and still return to his previous activity. If he went to establish his Shabbat residence in order to adjust his Shabbat limit for an optional purpose, rather than in order to fulfill a commandment, he should return immediately to remove his leaven.

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: