There are three witnesses that testify for one another: The Jewish people, Shabbos and the Holy One, blessed be He.
The Jewish people and Hashem bear witness that the Shabbos was instituted as a day of rest.
The Jewish people and the Shabbos attest that Hashem is the Creator of the universe.
Hashem and the Shabbos testify that the Jews are Hashem’s children whom He has chose for Himself.
~ Tosfos Chagigah 3b