ויאמר ה’ במה אדע כי אירשנה (לך לך טו:ח)

The Gemara (ברכות ז)  teaches that from the day the world was created, no one called Hashem by the name “אדון” until Avraham Avinu came and used this name — א‑ד‑נ‑י.

Once, a Rav approached the Vilna Gaon zt’’l, asking him to give an endorsement for a siddur he had transcribed for publication. The Gaon looked through the siddur and noticed that the author explained that the reason we begin Shacharis with the praise “אדון עולם” is based on the Gemara mentioned above. Since Avraham Avinu composed the Shacharis prayer, it is particularly fitting to start with the name “אדון”, as Avraham himself instituted it.

The Gaon was pleased with this insight and wrote his endorsement, saying:
“Just for this word alone, it is worthy to print this siddur!”