Baruch Shem K’vod
In response to the congregation declaring the Sh’ma aloud, each congregant answers with the following verse in an undertone or whisper so that others cannot hear:
Blessed is the Name of His glorious kingdom forever and ever!
This verse comes not from the Torah, but from a story in the Talmud (Pesachim 56a). When Jacob/Israel was about to give blessings to his sons on his deathbed, he lost his ability to see the future as he had wanted to tell his children about when the Messiah would come. As he worried that he lost this ability due to his children’s lack of righteousness, he was heartened to hear them all declare the Sh’ma Yisrael – Hear, O Israel! – proclaiming the Oneness of God. As thanks, Jacob declared “Baruch Shem K’vod” in response.
The Talmud states that the response to the Sh’ma is said in a whisper as it is not a phrase from the Torah and a midrash tells us that it was praise sung only by the angels. We do recite Baruch Shem K’vod out loud on Yom Kippur when we consider ourselves on the same level as the angels due to our sincere repentance.