From the founders. I linked to an article yesterday that quotes both Mussolini and Gentile's collectivist underpinning of the idealogy, and blatant refutation of any capitalist associations. Mussolini left the Socialist party and established fascism as a nationalistic
version of replacement for his contemporary socialism.
The father of fascism himself literally said it. “Fascism is a form of socialism, in fact, it is its most viable form.”
fee.org
Edit: I'll add that I have no clue how anyone seriously pawned Fascism as a Right-wing ideology:
So, the right is generally known as anti-collectivist, prioritizing individual rights via limited government over the needs of society. Fascism was a collectivist/socialist philosophy that crushed personal human rights in favor of the authority and needs of the gov't/society.
So it was obviously a far-right ideology.