The Jewish People are your Partners
From Aggadah Book, Medrash Bereishit Rabba, Chapter 11
Rabbi Shimon the son of Yochai taught: Shabbat said to the Holy-One-Blessed-be-He: Master of the world, everyone has a partner and I have no partner. So the Holy-One-Blessed-be-He told Shabbat: The Jewish people are your partner. And when the Israelites stood before Mount Sinai, the Holy-One-Blessed-be-He told them: Remember what I told Shabbat – the Jewish people are your partner. And this is what is referred to when it states: "Remember the day of Shabbat to sanctify it".