The Sefer HaChinukh presents the mitzvah of reading Shema, its twice-daily framework, and its purpose of affirming belief in God’s unity and accepting His sovereignty.
מִצְוַת קְרִיאַת שְׁמַע שַׁחֲרִית וְעַרְבִית – שֶׁנִּצְטַוִּינוּ לִקְרוֹת בְּכָל יוֹם עַרְבִית וְשַׁחֲרִית פָּסוּק אֶחָד מִן הַתּוֹרָה שֶׁבְּסֵדֶר זֶה, וְזֶהוּ (דברים ו ד) שְׁמַע יִשְׂרָאֵל יְיָ אֱלֹהֵינוּ יְיָ אֶחָד, וְעַל פָּסוּק זֶה נֶאֱמַר (שם) וְדִבַּרְתָּ בָּם בְּשִׁבְתְּךָ בְּבֵיתֶךָ וּבְשָׁכְבְּךָ וּבְקוּמֶךָ, וּבָא הַפֵּרוּשׁ עַל זֶה (ברכות י, ב) בְּשָׁעָה שֶׁבְּנֵי אָדָם שׁוֹכְבִים וּבְשָׁעָה שֶׁבְּנֵי אָדָם קָמִים, וְקָא מַשְׁמַע לְהוּ לְרַבָּנָן (שם), שֶׁבְּשָׁעָה שֶׁבְּנֵי אָדָם שׁוֹכְבִין יִקָּרֵא כָּל הַלַּיְלָה עַד שֶׁיַּעֲלֶה עַמּוּד הַשַּׁחַר, וּכְעִנְיָן שֶׁכָּתוּב (ויקרא כו ו) וּשְׁכַבְתֶּם וְאֵין מַחֲרִיד. וְכֵן לֹא יִשְׁכַּב עַד יֹאכַל טֶרֶף (במדבר כג כד). דְּמַשְׁמַע כָּל שְׁעַת שְׁכִיבָה. וְעוֹד שֶׁבְּנֵי אָדָם חֲלוּקִים הֵם בְּמִדּוֹתָם בְּעִנְיַן הַשְּׁכִיבָה, יֵשׁ מֵהֶם שֶׁאֵינָם שׁוֹכְבִים עַד חֲצִי הַלַּיְלָה, וְיֵשׁ עַד סוֹפָהּ, וְיֵשׁ שֶׁשּׁוֹכְבִים מִיָּד בִּתְחִלַּת הַלַּיְלָה, וּמִפְּנֵי כֵן אָמְרוּ (שם), שֶׁזְּמַן קְרִיאַת שְׁמַע בָּעֲרָבִין מִשָּׁעָה שֶׁהַכֹּהֲנִים נִכְנָסִין לֶאֱכֹל בִּתְרוּמָתָן, דְּהַיְנוּ צֵאת הַכּוֹכָבִים, עַד שֶׁיַּעֲלֶה עַמּוּד הַשַּׁחַר, וְשָׁעָה שֶׁבְּנֵי אָדָם קָמִים מַשְׁמַע לְהוּ מִתְּחִלַּת הַיּוֹם, כְּלוֹמַר, כְּשֶׁהַבֹּקֶר אוֹר כְּשֶׁאָדָם מַכִּיר אֶת חֲבֵרוֹ בְּרִחוּק אַרְבַּע אַמּוֹת, עַד שָׁלֹשׁ שָׁעוֹת שְׁלֵמוֹת (רמב"ם שם א יא). וְלָא מַשְׁמַע לְהוּ הַקִּימָה כָּל הַיּוֹם כְּמוֹ הַשְּׁכִיבָה, שֶׁאֵין דֶּרֶךְ אֶחָד מִבְּנֵי אָדָם שֶׁהוּא בָּרִיא שֶׁיָּקוּם מִמִּטָּתוֹ בְּסוֹף הַיּוֹם אוֹ אֲפִלּוּ בְּאֶמְצָעוֹ. וְאָמְרוּ זִכְרוֹנָם לִבְרָכָה (שם ט, ב) בִּקְרִיאַת שְׁמַע דְּשַׁחֲרִית דְּמִכָּל מָקוֹם מִכָּאן וְאֵילָךְ, כְּלוֹמַר, מִסּוֹף שָׁלֹשׁ שָׁעוֹת עַד סוֹף הַיּוֹם, מִי שֶׁלֹּא קָרָא לֹא הִפְסִיד, שֶׁלֹּא יוּכַל לִקְרוֹתָהּ עִם בִּרְכוֹתֶיהָ. מִדִּינֵי הַמִּצְוָה. מָה שֶׁאָמְרוּ זִכְרוֹנָם לִבְרָכָה (שם) שֶׁחִיּוּב פָּסוּק רִאשׁוֹן מִן הַפָּרָשָׁה הוּא מִן הַתּוֹרָה, כְּמוֹ שֶׁאָמַרְנוּ, אֲבָל חֲכָמִים חִיְּבוּנוּ לִקְרוֹת שָׁלֹשׁ פָּרָשִׁיּוֹת, שֶׁהֵן שְׁמַע וְהָיָה אִם שָׁמוֹעַ, וַיֹּאמֶר. וּמַקְדִּימִין (רמב"ם שם א, ב) לִקְרוֹת פָּרָשַׁת שְׁמַע שֶׁיֵּשׁ בָּהּ צִוּוּי עַל יִחוּד הַשֵּׁם יִתְבָּרַךְ וְאַהֲבָתוֹ וְתַלְמוּד תּוֹרָתוֹ, שֶׁהוּא הָעִקָּר הַגָּדוֹל שֶׁהַכֹּל תָּלוּי בּוֹ, וְאַחֲרֶיהָ פָּרָשַׁת וְהָיָה אִם שָׁמֹעַ, שֶׁיֵּשׁ בָּהּ צִוּוּי עַל שְׁאָר הַמִּצְוֹת כֻּלָּן, וְאַחֲרֶיהָ פָּרָשַׁת צִיצִית, שֶׁגַּם הִיא יֵשׁ בָּהּ צִוּוּי עַל זְכִירַת כָּל הַמִּצְוֹת, וְאִם כֵּן, בֶּאֱמֶת, בִּהְיוֹת הָאָדָם זוֹכֵר אֵלֶּה בְּכָל יוֹם פַּעַם אַחַת וּבְכָל לַיְלָה פַּעַם אַחֶרֶת בְּכַוָּנָה, יִנָּצֵל מִן הָעֲבֵרוֹת עַל כָּל פָּנִים אִם יֵשׁ דַּעַת בּוֹ.
The law of the recitation of Shema morning and evening: That we were commanded every day, morning and evening, to read one verse from the Torah in this Order, and that is “Hear Israel, the Lord is our God, the Lord is one” (Deuteronomy 6:4). And about this verse is it stated (Deuteronomy 6:7), “and you will speak in them in your sitting in your home... in your lying down and in your rising up.” And the explanation about this comes (Berakhot 10b) [that it is] at the time that people lie down and at the time that people get up. And it is established to us for the Rabbis (Berakhot 10b) that all of the night until the dawn rises is called the time that people lie down — and like the matter that is written (Leviticus 26:6), “and you will lie down and there is no one that makes [you] tremble”; and so [too], “it does not lie down until it eats prey” (Numbers 23:24) — since all the time of its lying down is implied. And also that people are divided in their attributes regarding lying down. There are those that do not lie down until half of the night, and some [not] until its end, and there are some that lie down immediately at the beginning of the night. And because of this, they said (Berakhot 10b) that the time of the recitation of Shema at night is from the time that the priests retire to eat their priestly tithe — which is the coming out of the stars — until the dawn rises. And the time that people rise up was understood [by] them [to be] from the beginning of the day — meaning to say when the morning is light [enough] that a man can recognize his fellow from the distance of four ells — until three full hours (Mishneh Torah, Laws of Reading the Shema 1:11). And rising up was not understood by them to be all of the day, like lying down; as it is not the way of any person that is healthy to rise up from his bed at the end of the day, or even its middle. And they, may their memory be blessed, said (Berakhot 9b) about the recitation of Shema of the morning that, in any case, from here onward — meaning from the end of three hours until the end of the day — he who did not read [it] did not lose [out]; however, he may not read it with its blessings. From the laws of the commandment is that which they, may their memory be blessed, said (Berakhot 13b) that the obligation of the first verse of the section is from Torah writ — as we have said — but [it is] the Sages that obligated us to read the three sections, which are “Shema,” “And it shall be if you listen” [and] “And He said.” And we begin with reading the section of Shema which has the command about unification of God, may He be blessed, His love and the study of His Torah, which is the great foundation that everything is dependent upon. And after it is the section of “And it shall be if you listen,” which has the command about all the other commandments. And after it is the section of fringes (tsitsit) that also has the command about the remembrance of all the commandments (Mishneh Torah, Laws of Reading the Shema 1:2). And if so, with a person remembering these [things] with intention once every day and another time every night, he will be saved from sin — in any case, if he is intelligent.