Before the Bris bein ha’Besarim [Covenant Between the Pieces], Avram referred to Hashem by the term spelled out “Alef Daled Nun Yud” (from the root word “Adon”, meaning master). The Talmud [Berachos 7b] states that this was the first time in the history of the world that any human being referred to Hashem by the title of ‘Master’.
There was a Maggid [itinerant preacher] who wrote a commentary on the prayer book and brought it to the Vilna Gaon for his approval (haskama). In his commentary, the author advanced the theory that the reason the Siddur begins with the prayer Adon Olam (Master of the World...) is because the morning prayer (Shacharis) was the prayer originated by Avraham, and the Talmud states that Avraham was the first person to use the term Adon-ai in referring to Hashem.
The Gaon commented that the whole commentary on the Siddur was worthy of being published just for the sake of this one insight.
