Full Idea
'Being' is unprovable, because there is no 'being'. The concept of being is formed out of the opposition to 'nothingness'.
Gist of Idea
There is no 'being'; it is just the opposition to nothingness
Source
Friedrich Nietzsche (Unpublished Notebooks 1884-85 [1884], 25[185])
Book Reference
Nietzsche,Friedrich: 'Fragments from 1884-85 (v 15)', ed/tr. Loeb,P.S./Tinsley,D.F. [Stanford 2022], p.54
A Reaction
Presumably a comment on Hegel's most basic idea. I find both thoughts bewildering. 'Being' is just a generalised (and unhelpful) way of referring to the self-evident existence of stuff.