אורח חיים קמ״וOrach Chayim 146

Se’if 1א׳

אָסוּר לָצֵאת וּלְהַנִּיחַ סֵפֶר תּוֹרָה כְּשֶׁהוּא פָּתוּחַ, אֲבָל בֵּין גַּבְרָא לְגַבְרָא, שַׁפִּיר דָּמֵי.

To Not Speak at the Time of the Torah Reading. Containing 4 S'ifim: It is forbidden to exit [the synagogue] and leave the Torah while it is open. But between one person's [aliyah] and another, it is permitted.

Se’if 2ב׳

כֵּיוָן שֶׁהִתְחִיל הַקּוֹרֵא לִקְרוֹת בְּסֵפֶר תּוֹרָה, אָסוּר לְסַפֵּר אֲפִלּוּ בְּדִבְרֵי תּוֹרָה, אֲפִלּוּ בֵּין גַּבְרָא לְגַבְרָא, וַאֲפִלּוּ אִם הִשְׁלִים הוּא הַפָּרָשָׁה; וְיֵשׁ מַתִּירִים לִגְרֹס (פי' לִלְמֹד) בְּלַחַשׁ, וְיֵשׁ אוֹמְרִים שֶׁאִם יֵשׁ י' דְּצַיְיתֵי (פי' הַמְשִׂימִין לִבָּם) לַסֵפֶר תּוֹרָה, מֻתָּר לְסַפֵּר בְּדִבְרֵי תּוֹרָה (בֵּית יוֹסֵף בְּשֵׁם מַהֲרִי"א); וְיֵשׁ מַתִּירִים לְמִי שֶׁתּוֹרָתוֹ אֻמָּנוּתוֹ; וְיֵשׁ מַתִּירִים לְמִי שֶׁקּוֹדֵם שֶׁנִּפְתַּח סֵפֶר תּוֹרָה מַחֲזִיר פָּנָיו וּמַרְאֶה עַצְמוֹ שֶׁאֵינוֹ רוֹצֶה לִשְׁמֹעַ סֵפֶר תּוֹרָה אֶלָּא לִקְרוֹת, וּמַתְחִיל לִקְרוֹת; וְלִקְרוֹת שְׁנַיִם מִקְרָא וְאֶחָד תַּרְגּוּם בִּשְׁעַת קְרִיאַת הַתּוֹרָה, שָׁרֵי. וְכָל זֶה אֵינוֹ עִנְיָן לְפָרָשַׁת זָכוֹר (דְּבָרִים כה, יז יט) וּפָרָשַׁת פָּרָה (בַּמִּדְבָּר יט, א כב), שֶׁהֵם בַּעֲשָׂרָה מִדְּאוֹרַיְיתָא, שֶׁצָּרִיךְ לְכַוֵּן וּלְשָׁמְעָם מִפִּי הַקּוֹרֵא; וְהַנָּכוֹן שֶׁבְּכָל הַפָּרָשִׁיּוֹת רָאוּי לִמְדַקְדֵּק בִּדְבָרָיו לְכַוֵּן דַּעְתּוֹ וּלְשָׁמְעָם מִפִּי הַקּוֹרֵא.

Once the reader began reading from the Torah scroll, it is forbidden to speak even words of Torah, and even between one person [i.e. aliyah] and the another, and even if one already finished the parasha. And there are those who permit to "grind out" (meaning: to learn) quietly. And there are those who say that if there are ten who are listening (meaning: they are paying attention) to the Torah scroll, then it is permitted to discuss (words of Torah) (Beit Yosef in the name of Mahari"a). And there are those who permit it for those for whom Torah is their profession. And there are some who permit it for one who turns one's face away before they open the Torah scroll, who makes self appear that one does not want to listen to the Torah scroll, but rather [wants] to read [i.e. learn], and begins to read [i.e. learn]. And to read "shnayim mikrah, v'echad targum" [i.e., the practice of "reading each verse twice, and the Aramaic translation once"] during the Torah reading is permitted. And all this does not apply to Parashat Zachor and Parashat Parah which must be read since they [require a quorum of] ten [people] as [obligations] from the Torah that require one to concentrate and hear [them] from the mouth of the reader. And it is proper that for all the parshiot [throughout the year], it is fitting to be scrupulous with the reader's words and for one to pay attention and hear them from the reader.

Se’if 3ג׳

אָסוּר לְסַפֵּר כְּשֶׁהַמַּפְטִיר קוֹרֵא בַּנָּבִיא, עַד שֶׁיַּשְׁלִים כְּמוֹ בְּסֵפֶר תּוֹרָה.

It is forbidden to speak when the Maftir is reading from the Prophets until he finishes, just like with the Torah scroll.