It Shall Be
Vehaya im-shamo-a tishm'u el mitzvotai asher anochi metza-veh etchem hayom le-ahavah et Hashem Elokeichem ul-avdo b'chol-levavchem uvchol nafshechem
And it will come to pass that if you continually hearken to My commandments that I command you today, to love Hashem, your G-d, and to serve Him, with all your heart and with all your soul -
Venatati metar artzechem be-ito yoreh u-malkosh ve-asafta deganecha vesiroshcha veyitzharecha
then I will provide rain for your land in its proper time, the early and late rains, that you may gather in your grain, your wine, and your oil.
Venatati esev besadecha livhemtecha ve-achalta vesava-ta
I will provide grass in your field for your cattle and you will eat and be satisfied.
This passage of the Shema' is from Devarim 11:13-21 and begins with the word Vehaya (It shall be).
In the ancient Temple, after the Shema' was read, all three paragraphs were then recited aloud by the Kohanim following the daily morning offerings. (Tamid 5:1). The people assembled in the Temple courtyard did not join in this reading, but on hearing the first sentence of the Shema', they responded with:
Baruch shem kevod malchuto l'olam va-ed!
Blessed is the Name of His Glorious Kingdom forever and ever!This response is not a quotation of any biblical verse though it bears some similarities to a verse in Tehillim 72:19. This became the response to all blessings recited by the Kohanim in the Temple, including the Priestly Blessing. It was used in lieu of "Amein." It was also the response to the declaration, "Hear O YIsarel, Hashem is our G-d, Hashem is One!" (Taanit 16b; Yoma 6:2, 3:8).
This response was later transferred to the Synagogue as a standard part of the Shema'. In order to indicate that this sentence is not part of the Biblical passage of the Shema', it is customary to say it in a more subdued voice than the rest of the Shema' (Pesachim 56a). The only exception to this is on Yom Kippur when it is deliberately said aloud.
Sources:
Artscroll Siddur
To Pray As A Jew - Donin