מדרש על שמות 34:23

(Ibid.) "the L-rd our G-d": Why is this written? Is it not written thereafter "the L-rd is One"? It is upon us that His name is reposed most distinctly (thus, "our G-d"). Similarly, (Shemoth 34:23) "Three times in the year shall all of your males appear before the L-rd, Hashem (Yod-keh-vav-keh), the G-d of Israel." Is it not already written "before the L-rd"? Why, then, "the G-d of Israel"? It is upon us that His name is reposed most distinctly.