הלכה על ישעיהו 6:3

One should not say (to oneself): "I am now in an inner room - who can see me?", for the Holy One Blessed Be He, "His glory fills the whole earth" (Isaiah 6:3).

After that, U-va Le-Tziyon, also called Kedusha De-sidra, is recited. It contains the verses “Kadosh, kadosh, kadosh…” (Yeshayahu 6:3), “Barukh kevod Hashem mi-mkomo” (Yeḥezkel 3:12), and “Hashem yimlokh le-olam va’ed” (Shemot 15:18). The uniqueness of this Kedusha is that the verses are recited along with their Aramaic translations. The Sages ordained their recitation (even though they were already recited in Yotzer Or and in Kedusah of Ḥazarat Ha-shatz) so that every person praying would learn some verses of the Prophets every day. The verses are translated into Aramaic, so that the whole nation, which was fluent in Aramaic at that time the prayer was ordained, would understand their meaning.

Kedusha is recited in the third berakha of Ḥazarat Ha-shatz. The essence of the Kedusha is the congregation’s response with the verses: “Kadosh, kadosh, kadosh, Hashem Tzevakot, melo kol ha-aretz kevodo” (“Holy, holy, holy is the Lord of hosts; the whole world is filled with His glory”; Yeshayahu 6:3), “Barukh kevod Hashem mi-mkomo” (“Blessed be the Lord’s glory from His place”), and “Yimlokh Hashem Le-olam…” (“The Lord shall reign forever…; Tehilim 146:10). The custom today, based on Arizal, is for the congregation to recite the connecting passages as well. The ḥazan then repeats them aloud, and the congregation responds with the verses of the Kedusha (MB 125:2; Kaf Ha-ḥayim 2).