Baruch Shem Kivod Malchuto II
The Rabbis say, when Moshe ascended to heaven he heard the ministering angels saying to God, “Blessed be the Name of His glorious kingdom for ever and ever.” Moshe brought this declaration down to Israel. And why do not Israel make this declaration aloud?
Rabbi Assi replied, this can be compared to a man who stole jewelry from the royal palace which he gave to his wife, telling her, “Do not wear these in public, but only in the house.” But on the Day of Atonement when Israel are as pure as the ministering angels they do recite it publicly. (Devarim Rabbah 2:36)