Ramban explains the various names of Hashem and levels of prophetic revelation (explaining pessukim 2-4 all in one comment). Right before the end of this section (paragraph 4 if using HaMeor layout) he writes: וכבר רמזתי פירוש "השמים - where would this be? Back in Bereishis? I don't see any contextual reference.
It is a reference to Breishit 1:8 according to sefaria.