Sifrei Devarim Paragraph 167
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(דברים יח ה) כִּי בוֹ בָחַר ה׳ אֱלֹהֶיךָ מִכָּל שְׁבָטֶיךָ – מַגִּיד שֶׁאֵין שֵׁרוּת כָּשֵׁר בּוֹ אֶלָּא מֵעוֹמֵד; הָא אִם יָשַׁב וְשֵׁרֵת – עֲבוֹדָתוֹ פְּסוּלָה.
(Devarim 18:5) "For in him (the Cohein) did the L-rd your G-d choose from all of your tribes to stand to serve in the name of the L-rd": We are hereby taught that his service is valid only standing, and if he sat and served, his service is invalid.
הוּא וּבָנָיו כָּל הַיָּמִים – בֵּין בָּאָרֶץ וּבֵין בְּחוּצָה לָאָרֶץ.
"he and his sons all of the days": both in the time of the Temple and not in the time of the Temple; both in Eretz Yisrael and outside of it.