There is a trick for figuring this out. You go to Google Scholar and try out a series of custom ranges.
Doing that, I discovered the following:
I argue that their work, though powerful and brilliant in many ways, relies on what I call gender essentialism-the notion that a unitary, "essential" women's experience can be isolated and described independently of race, class, sexual orientation, and other realities of experience.
Harris, Angela P. "Race and essentialism in feminist legal theory." Stanford law review (1990): 581-616.
Essentialism is implicit in analogies between sex and race. Angela Harris explains gender
essentialism as "[t]he notion that there is a mono- lithic 'women's experience' that can be described
independent of other facets of experience like race, class, and sexual orientation."
Grillo, Trina, and Stephanie M. Wildman. "Obscuring the importance of race: The implication of making comparisons between racism and sexism (or other-isms)." Duke Law Journal (1991): 397-412.
The Harris 1990 article is cited by 2503 other works. Before making a claim in print, of course one would want to do a little more research, but Harris looks very likely!