… is from page 288 of Kristian Niemietz’s 2019 book, Socialism: The Failed Idea that Never Dies:
This is, in essence, the fallacy that socialists commit in their definition of real socialism. They define ‘real’ socialism in terms of outcomes they would like to see. When a socialist experiment does not produce those outcomes, it retroactively becomes unreal. Since socialism never produces those outcomes, all socialist experiments sooner or later become unreal. This is the deeper meaning behind the old adage that ‘real socialism has never been tried’.