Mishnah 1

הָיָה קוֹרֵא בַתּוֹרָה, וְהִגִּיעַ זְמַן הַמִּקְרָא: אִם כִּוֵּן לִבּוֹ – יָצָא, וְאִם לָאו – לֹא יָצָא. בַּפְּרָקִים – שׁוֹאֵל מִפְּנֵי הַכָּבוֹד וּמֵשִׁיב, וּבָאֶמְצַע – שׁוֹאֵל מִפְּנֵי הַיִּרְאָה וּמֵשִׁיב; דִּבְרֵי רַבִּי מֵאִיר. רַבִּי יְהוּדָה אוֹמֵר: בָּאֶמְצַע – שׁוֹאֵל מִפְּנֵי הַיִּרְאָה, וּמֵשִׁיב מִפְּנֵי הַכָּבוֹד; בַּפְּרָקִים – שׁוֹאֵל מִפְּנֵי הַכָּבוֹד, וּמֵשִׁיב שָׁלוֹם לְכָל אָדָם.

One who was reading [the Shema] in the Torah, and it came time to recite [the Shema]: If he had intent in his heart, he has fulfilled his obligation. And if not, he has not fulfilled his obligation. At the breaks [between sections of the Shema], one may greet out of honor and return a greeting, And in the middle [of a section], one may greet out of fear and return a greeting, these are the words of Rabbi Meir. Rabbi Yehuda says: in the middle, one may greet out of fear and return a greeting out of honor, and at the breaks, one may greet out of honor and return a greeting to any person.