1.The Mitzvah of Keriat Shema (The Recital of Shema)
It is a positive biblical commandment to recite Shema at night and in the morning, as it is written (Deuteronomy 6:7), "And you shall say them… when you lie down and when you get up." "When you lie down" denotes nighttime, and "when you get up" refers to morning.
We read three paragraphs, the first: "Shema Yisrael" ("Hear O Israel") (Deuteronomy 6:4-9), with which we accept the yoke of Heaven, and which discusses the unity and love of Hashem. The second is "V’hayah Im Shamo’a" ("And if you follow") (Deuteronomy 11:13-21), which includes the acceptance of the yoke of the mitzvot. The third is "Vayomer" ("And He said") (Numbers 15:37-41), including a command to remember the mitzvot through the commandment of tzitzit, and mention of the Exodus from Egypt.
The Chachamim arranged the Shema paragraph to precede V’hayah Im Shamo’a so that a person will first accept the yoke of Heaven, and only afterwards accept the yoke of the mitzvot. They also placed V’hayah Im Shamo’a which involves a general mandate to keep all the mitzvot, including those performed during both day and night, before the Vayomer paragraph, which discusses the mitzvah of tzitzit, performed only during the day (Berachot 13a).
In the opinion of some Rishonim, the biblical commandment is just to recite the verse "Shema Yisrael." According to them, this is the meaning of (Deuteronomy 6:6-7), "These words I am commanding you today must be in your heart…when you lie down and when you get up." However, the Chachamim instituted reciting the entire three paragraphs.
It can be explained, that the essence of the mitzvah of Keriat Shema is indeed that a person accepts upon himself the yoke of Heaven, and for that reason, even one who only recites the first verse fulfills the biblical commandment. Yet, the more a person enhances his acceptance of the yoke of Heaven, the more completely he fulfills the biblical commandment. Therefore, the Chachamim instituted the recital of all three paragraphs, for within them are the fundamentals of faith, the acceptance of the yoke of the mitzvot, and the remembrance of all the mitzvot through the mitzvah of tzitzit. Hence, in actuality, we fulfill the biblical mitzvah by reciting all three paragraphs. 1
2. Remembering the Exodus
It is a biblical commandment to remember the Exodus every day, as it says (Deuteronomy 16:3), "Therefore you will remember the day you left Egypt all the days of your life." The Chachamim learn from the word "all" (kol) that the mitzvah to remember the Exodus is performed both during the day and at night (Berachot 12b). This mitzvah can be fulfilled by reciting any verse that discusses leaving Egypt, or by mentioning the Exodus in one’s own words.
The Chachamim instituted reciting the Vayomer paragraph in order to fulfill the mitzvah of remembering the Exodus from Egypt. Therefore, there are two reasons why Vayomer was incorporated into the recital of the Shema. First, it mentions the mitzvah of tzitzit that reminds us of all the mitzvot. Second, it talks about the Exodus from Egypt. It is therefore customary to say Vayomer even at night, for although there is no need to mention the mitzvah of tzitzit then, we say it to remember the Exodus (see Berachot 14b, and Kesef Mishneh, Hilchot Keriat Shema, chapter 1: 2-3).
There is a difference between the mitzvah of Keriat Shema and the mitzvah of remembering the Exodus from Egypt. The mitzvah of Keriat Shema can only be fulfilled within the first three hours of the day because that is the time we wake up, whereas the daytime mitzvah of remembering the Exodus can be performed throughout the entire day. However, following the enactment of the Chachamim, we fulfill the mitzvah of remembering the Exodus with the recital of Shema, and if the time to say Shema has passed, a person can remember the Exodus from Egypt by reciting the berachah of "Emet V’Yatziv." If four hours have passed, one may fulfill the mitzvah by saying the Vayomer paragraph or by remembering the Exodus in another way (Mishnah Berurah 58:27, 67:3; see earlier in this book 11:11; and see the laws of Ma’ariv, further in this book chapter 25, end of note 3).
^ 1.The question is to what the command, "Speak of them…when you lie down and when you get up," applies. According to the Ramban, Ra’ah, Rashba, Ritva, Meiri, Rashbatz, Rabbi Yehudah HaChassid, and Beit Yosef 46:9, it applies only to the first verse. This is implied in the Shulchan Aruch 63:4, who writes only concerning the first verse, that if a person did not have kavanah, he does not fulfill his obligation. In the opinion of Talmidei Rabbi Yonah and the Yere’im, the biblical mitzvah applies to the whole first paragraph. However, the words in V’hayah Im Shamo’a, "Speak of them…when you lie down and when you get up" (Deuteronomy 11:19), refer to the mitzvah of learning Torah during the day and at night. The Pri Chadash 67 maintains that the biblical mitzvah is the recital of both the first two paragraphs. See Yabia Omer 8:6:4. Also see Aruch HaShulchan 58:15, who infers from a few Rishonim that it is a biblical commandment to recite all three paragraphs. The Mabit explains in Kiryat Sefer that like in the mitzvah of learning Torah, a person who learned one verse already fulfilled the mitzvah, but the more he learns, the more he fulfills the mitzvah, so it is regarding the recital of Shema. There is a similar explanation in Yad Peshutah in his introduction to Hilchot Keriat Shema.