Berakhot 12B

רב ששת, כי כרע ⁠— כרע כחיזרא. כי קא זקיף ⁠— זקיף כחיויא.

With regard to bowing, the Talmud relates: When Rav Sheshet bowed he bowed all at once, like a cane, without delay. When he stood upright he stood upright like a snake, lifting himself slowly, demonstrating that the awe of God was upon him in the manner that he bowed and stood upright (HaBoneh).

ואמר רבה בר חיננא סבא משמיה דרב כל השנה כולה אדם מתפלל ״האל הקדוש״, ״מלך אוהב צדקה ומשפט״, חוץ מעשרה ימים שבין ראש השנה ויום הכפורים, שמתפלל ״המלך הקדוש״, ו״המלך המשפט״.

And, with regard to the formulation of the blessings, Rabba bar Ḥinnana Sava said in the name of Rav: Throughout the year a person prays and concludes the third blessing of the Amida prayer with: The holy God, and concludes the blessing regarding the restoration of justice to Israel with: King who loves righteousness and justice, with the exception of the 10 days between Rosh HaShana and Yom Kippur, the 10 Days of Atonement. These days are comprised of Rosh HaShana, Yom Kippur, and the 7 days in between, when one emphasizes God’s sovereignty, and so when he prays he concludes these blessings with: The holy King and: The King of justice, i.e., the King who reveals Himself through justice.

ורבי אלעזר אמר אפילו אמר ״האל הקדוש״⁠ — יצא. שנאמר: ״ויגבה ה׳ צבאות במשפט והאל הקדוש נקדש בצדקה״. אימתי ״ויגבה ה׳ צבאות במשפט״⁠ — אלו עשרה ימים שמראש השנה ועד יום הכפורים, וקאמר ״האל הקדוש״.

In contrast, R' Elazar said that one need not be exacting, and even if he said: The holy God during those 10 days, he fulfilled his obligation, as it is stated: “And YHWH of Hosts is exalted through justice, and the holy God is sanctified through righteousness” (Isaiah 5:16). The Talmud explains: When is it appropriate to describe God with terms like: And YHWH of Hosts is exalted through justice? It is appropriate when God reveals Himself through justice, during the 10 days between Rosh HaShana and Yom Kippur, yet the verse says: The holy God. This appellation sufficiently underscores God’s transcendence, and there is no need to change the standard formula.