Talmud Bavli, Pesachim
Pesachim 51a
The Gemara discusses the principle that one who travels from a more lenient place to a stricter one must observe the stricter customs publicly to avoid appearing to act differently — but the full personal obligation depends on intent to return, establishing the foundational framework for the Israeli-abroad question.
וְרַבָּה בַּר בַּר חָנָה לֵית לֵיהּ הָא דִּתְנַן נוֹתְנִין עָלָיו חוּמְרֵי הַמָּקוֹם שֶׁיָּצָא מִשָּׁם וְחוּמְרֵי הַמָּקוֹם שֶׁהָלַךְ לְשָׁם? אָמַר אַבָּיֵי: הָנֵי מִילֵּי מִבָּבֶל לְבָבֶל, וּמֵאֶרֶץ יִשְׂרָאֵל לְאֶרֶץ יִשְׂרָאֵל. אִי נָמֵי, מִבָּבֶל לְאֶרֶץ יִשְׂרָאֵל. אֲבָל מֵאֶרֶץ יִשְׂרָאֵל לְבָבֶל — לָא. כֵּיוָן דַּאֲנַן כַּיְיפִינַן לְהוּ, עָבְדִינַן כְּווֹתַיְיהוּ. רַב אָשֵׁי אָמַר: אֲפִילּוּ תֵּימָא מֵאֶרֶץ יִשְׂרָאֵל לְבָבֶל, הָנֵי מִילֵּי הֵיכָא דְּאֵין דַּעְתּוֹ לַחֲזוֹר, וְרַבָּה בַּר בַּר חָנָה דַּעְתּוֹ לַחֲזוֹר הֲוָה. אֲמַר לֵיהּ רַבָּה בַּר בַּר חָנָה לִבְנֵיהּ: בְּנִי, לֹא תֹּאכַל לֹא בְּפָנַי וְלֹא שֶׁלֹּא בְּפָנַי. אֲנִי שֶׁרָאִיתִי אֶת רַבִּי יוֹחָנָן שֶׁאָכַל — כְּדַי הוּא רַבִּי יוֹחָנָן לִסְמוֹךְ עָלָיו בְּפָנָיו וְשֶׁלֹּא בְּפָנָיו. אַתָּה לֹא רָאִיתָ אוֹתוֹ, לֹא תֹּאכַל בֵּין בְּפָנַי בֵּין שֶׁלֹּא בְּפָנַי.
The Gemara asks: And is Rabba bar bar Ḥana, who was lenient with regard to a matter that is prohibited, not in agreement with that which we learned in the mishna: When one travels from one place to another, the Sages impose upon him the stringencies of the place from which he left and the stringencies of the place to which he went? Abaye said: That applies when one travels from one place in Babylonia to another place in Babylonia, or from one place in Eretz Yisrael to another place in Eretz Yisrael, or alternatively, from Babylonia to Eretz Yisrael. However, when traveling from Eretz Yisrael to Babylonia, no, this principle does not apply. Since we, the residents of Babylonia, are subordinate to them in terms of halakha, we act in accordance with their custom, but a resident of Eretz Yisrael is not required to follow the Babylonian custom. Rav Ashi said: Even if you say that when one travels from Eretz Yisrael to Babylonia, he is required to act stringently in accordance with the local custom, this applies only when his intent is not to return. One is required to adopt the local customs when permanently settling in a new location. However, as Rabba bar bar Ḥana’s intent was to return to Eretz Yisrael, his point of origin, he continued to follow the custom of Eretz Yisrael. The Gemara relates that Rabba bar bar Ḥana said to his son: My son, you live in Babylonia. Therefore, do not eat this fat, neither when you are in my presence nor when you are not in my presence. I, who saw Rabbi Yoḥanan eat this fat, can say that Rabbi Yoḥanan is worthy for one to rely upon him both in his presence and not in his presence. You did not see him. Therefore, do not eat it, neither when you are in my presence nor when you are not in my presence, since you may not rely upon my opinion alone in this matter.
Why it matters — This sugya is the primary Talmudic source for the rule that travelers must respect local customs, which is the basis for restricting Israeli visitors from performing melachah openly on Yom Tov Sheni.