(א) וַיִּקְרָ֥א יַעֲקֹ֖ב אֶל־בָּנָ֑יו וַיֹּ֗אמֶר הֵאָֽסְפוּ֙ וְאַגִּ֣ידָה לָכֶ֔ם אֵ֛ת אֲשֶׁר־יִקְרָ֥א אֶתְכֶ֖ם בְּאַחֲרִ֥ית הַיָּמִֽים׃ (ב) הִקָּבְצ֥וּ וְשִׁמְע֖וּ בְּנֵ֣י יַעֲקֹ֑ב וְשִׁמְע֖וּ אֶל־יִשְׂרָאֵ֥ל אֲבִיכֶֽם׃ (ג) רְאוּבֵן֙ בְּכֹ֣רִי אַ֔תָּה כֹּחִ֖י וְרֵאשִׁ֣ית אוֹנִ֑י יֶ֥תֶר שְׂאֵ֖ת וְיֶ֥תֶר עָֽז׃ (ד) פַּ֤חַז כַּמַּ֙יִם֙ אַל־תּוֹתַ֔ר כִּ֥י עָלִ֖יתָ מִשְׁכְּבֵ֣י אָבִ֑יךָ אָ֥ז חִלַּ֖לְתָּ יְצוּעִ֥י עָלָֽה׃ {פ}
(1) And Jacob called his sons and said, “Come together that I may tell you what is to befall you in days to come. (2) Assemble and hearken, O sons of Jacob; Hearken to Israel your father: (3) Reuben, you are my first-born, My might and first fruit of my vigor, Exceeding in rank And exceeding in honor. (4) Unstable as water, you shall excel no longer; For when you mounted your father’s bed, You brought disgrace—my couch he mounted!
What does the Shema have to do with this text?
שְׁמַע יִשְׂרָאֵל יהוה אֱלֹהֵֽינוּ יהוה אֶחָד:
Hear, O Israel: Adonai is our God, Adonai is One.
בָּרוּךְ שֵׁם כְּבוֹד מַלְכוּתוֹ לְעוֹלָם וָעֶד:
Blessed is God's Name, Whose glorious kingdom is forever and ever.
From this the Jews merited to say Kriat Shema, When Jacob called them together he asked if any of them had any complaint against Hashem. They responded, Hear Oh Israel [our father] just as you dont have any contention, neither do we; Hashem the Lord is The One and Only. Jacob responded by saying "blessed is the name of His kingdom for all eternity"
Bereishit Rabbah 98:4
“And Jacob called unto his sons, and said: ‘Gather yourselves together, that I may reveal to you…’” (Gen 49:1). What Jacob was about to reveal to his sons was when the time of redemption was to come. But the Presence departed from him. So he said, “Could it be, God forbid, that my bed has produced an unfit son—as happened to Abraham, my father's father, out of whom sprang Ishmael, or as happened to my father Isaac, out of him sprang Esau?” His sons assured him of their steadfastness: “‘Hear, O Israel [our father], The Lord is our God, the Lord alone' [Deut 6:4]. Just as in your heart there's only the One, so in our hearts there is only the One.” Then our father Jacob pronounced for the first time the benediction “Blessed be the name whose glorious kingdom is forever and ever.”
Hence—it is reported in the name of R. Samuel—every day, morning and evening, Jews say, “In the cave of Machpelah [where you are at rest], ‘Hear, O Israel [our father].’ What you commanded us we still practice: ‘The Lord is our God, the Lord alone.’”
How do we recite the second line of the Shema?
Pesachim 56a
What is the reason that we recite that passage: Blessed be the name of His glorious kingdom for ever and ever, even though it does not appear in the Torah? The Gemara answers: We recite it in accordance with that which Rabbi Shimon ben Lakish interpreted homiletically. As Rabbi Shimon ben Lakish said that it is written: “And Jacob called his sons and said, Gather around and I will tell you what will occur to you in the end of days” (Genesis 49:1). Jacob wanted to reveal to his sons when the complete redemption would arrive at the end of days (see Daniel 12:13), but the Divine Presence abandoned him, rendering him unable to prophesy. He said: Perhaps the Divine Presence has abandoned me because, Heaven forfend, one of my descendants is unfit, as was the case with my grandfather Abraham, from whom Ishmael emerged, and like my father Isaac, from whom Esau emerged. His sons said to him: Hear Israel, our father, the Lord is our God, the Lord is One. They said: Just as there is only one God in your heart, so too, there is only one in our hearts. At that moment Jacob our father said in praise: Blessed be the name of His glorious kingdom for ever and ever, as all his children were righteous. The Rabbis said: What should we do? Shall we recite this verse? But Moses our teacher did not say it in the Torah as part of Shema. Shall we not recite it? But Jacob said it. In order to resolve this dilemma they established that this passage should be recited surreptitiously....
Midrash Rabba to Deuteronomy
When Moses went up to the heavens, he heard the angels praise God by saying, “Blessed [is His] Name, etc.” When he returned to earth, he taught it to the Jewish People. We say it in an undertone so as not to offend the angels. On Yom Kippur however, when we are considered like angels, we are permitted to say ברוך שם aloud.
Rabbi Lawrence Kushner in My People's Prayerbook, v. 1. p.94.
"...the Barukh Shem which is not in the biblical text itself, but was added by the Rabbis as a congregational response. The Barukh Shem, [Ba'al Hatanya] says, is our attempt to bring back into this world the supernal unity spoken of in the first line. We have a vision of ultimate unity when we utter [the Shema]. And when we recite Barukh shem k'vod malkhuto l'olam va'ed, we try to bring that unity into everyday reality."