Va’ethanan 

ואתחנן

After reciting the verse “Hear Israel,” it is customary to add the words: “Blessed is the name of His glorious kingdom for ever and ever.” According to the Sages, that is how the angels praise God, and Moses heard it from them and taught the Israelites to say this as well. 

“Hear Israel.” The Sages say: When Moses ascended on High, he heard the ministering angels saying to the Holy One, blessed be He: Blessed is the name of His glorious kingdom for ever and ever, and he took awareness of this praise down with him and taught it to Israel. 

Why don’t the Israelites recite: Blessed is the name of His glorious kingdom for ever and ever, out loud, publicly? Rabbi Asi said: To what can this matter be compared? It can be compared to one who stole jewelry from a king’s palace. He gave it to his wife and said to her: Do not adorn yourself with it in public, but rather in your house. But on Yom Kippur, when the Jewish people are cleansed of sin and like angels, they recite it out loud: Blessed is the name of His glorious kingdom for ever and ever. <source>(<i>Devarim</i> <i>Rabba</i> 2) </source>