Full Idea
Leibniz said that the ontological argument does not prove God's existence, but only the God's existence is either necessary or impossible.
Clarification
Because it is a matter of pure reason
Gist of Idea
God's existence is either necessary or impossible
Source
report of Gottfried Leibniz (works [1690]) by Roger Scruton - Modern Philosophy:introduction and survey 13.5
Book Reference
Scruton,Roger: 'Modern Philosophy: introduction and survey' [Sinclair-Stevenson 1994], p.171