I have found in multiple sources that Rishonim refer to Shabbos as shabbos bereishis, and it is clear from context that they are referring to the weekly Shabbos as opposed to Shabbos of Creation.
For example, the Rambam says in Hilchos Kiddush Hachodesh 1:5
אין ראיית הירח מסורה לכל אדם כמו שבת בראשית שכל אחד מונה ששה ושובת בשביעי
And Rashi in parshas Behar 25:2 regarding shemita:
ושתבה הארץ שבת לה' - לשם ה', כשם שנאמר בשבת בראשית
What is the reason that they refer to it this way? Is there any primary source for this?