The Essence of the Mitzva of Shema

Hear, O Israel! יהוה is our God, יהוה alone.

You shall love your God יהוה with all your heart and with all your soul and with all your might.

Take to heart these instructions with which I charge you this day.

Impress them upon your children. Recite them when you stay at home and when you are away, when you lie down and when you get up.

Bind them as a sign on your hand and let them serve as a symbol on your forehead;

inscribe them on the doorposts of your house and on your gates.

— What is the “love” that is here commanded? The next verse tells us: והיו הדברים האלה THESE WORDS [WHICH I COMMAND THEE] SHALL BE [UPON THINE HEART] — for thereby thou wilt arrive at a recognition of the Holy One, blessed be He, and wilt cleave to His ways (Sifrei Devarim 33:1).

אשר אנכי מצוך היום [THESE WORDS] WHICH I COMMAND THEE THIS DAY — this day— they should not be in thine eyes as an antiquated דיוטגמא which no one minds, but as one newly given which everyone gladly welcomes (Sifrei Devarim 33:2; cf. our first Note on p. 248 in the Silbermann edition of Exodus). — The word דיוטגמא in this Midrash signifies: a royal command committed to writing.

AND THESE WORDS…SHALL BE. Those who deny the Rabbinic tradition say that the reference is to the Decalogue, and that it is in reference to the Decalogue that Scripture states, And thou shalt write them upon the door-posts (v. 9). However, the truth is that and these words…shall be refers to all of the commandments.

That is that He commanded us to read the recitation of Shema in the evening and the morning. And that is His saying, "and you shall speak about them" (Deuteronomy 6:4). And the regulations of this commandment have already been explained in Tractate Berakhot; and there (Berakhot 21a), it is explained that the recitation of Shema is [an obligation] from the Torah. And it is written in the Tosefta (Tosefta Berakhot 3:1), "Just like the Torah established [a set time for] the recitation of Shema, so too did the Sages establish a time for prayer." This means to say that the times of prayer are not from the Torah - though the actual obligation to pray is from the Torah, as we explained (Sefer HaMitzvot, Positive Commandments 5) - and the Sages, may their memory be blessed, arranged times for them. And this is the content of their saying (Berakhot 26b), "They established the prayers corresponding to the daily sacrifices" - meaning, that they fixed their times according to the times of the sacrifices. And women are not obligated in this commandment. (See Parashat Vaetchanan; Mishneh Torah, Reading the Shema 1.)

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 laying down and in your rising up." And the explanation about this comes (Berakhot 10b) [that it is] at the time that people lay 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 lay down - and like the matter that is written (Leviticus 26:6), "and you will lay down and there is no one that makes to tremble"; and so [too], "it does not lay down until it eats prey" (Numbers 23:24) - since all the time of its laying down is implied. And also that people are divided in their attributes regarding laying down. There are those that do not lay down until half of the night, and some [not] until its end, and there are some that lay 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 laying 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] that he not be able to read it with its blessings.

We [are obligated to] recite the Shema twice daily - in the evening and in the morning - as [Deuteronomy 6:7] states: "...when you lie down and when you rise" - i.e., when people are accustomed to sleep - this being the night - and when people are accustomed to rise, this being daytime.

And what is it that one recites? These three sections:
"Hear O Israel..." (Deuteronomy 6:4-9),

"And if you will listen..." (Deuteronomy 11:13-21),and

"And God said..." (Numbers 15:37-41).
We begin with the section of "Hear O Israel" since it contains [the concept of] the unity of God, [the commandment of] loving Him and the study of Torah, it being a fundamental principle upon which everything is based.
After it, [we read] "And if you will listen...," since it contains the imperative to fulfill the rest of the commandments, and finally the portion of tzitzit, since it also contains the imperative of remembering all the commandments.

The commandment of tzitzit is not obligatory at night. Nevertheless, we recite [the section describing] it at night because it contains mention of the exodus from Egypt.
We are commanded to mention the exodus both during the day and at night as [Deuteronomy 16:3] states: "In order that you shall remember the day of your leaving the land of Egypt all the days of your life."
Reading these three sections in this order constitutes the recitation of the Shema.