There is a phrase that is pretty commonly known to fans of H.P. Lovecraft's Cthulhu mythos (whether or not they've actually read the stories or not):
Iä! Iä! Cthulhu fhtagn!
Sometimes a longer version of the phrase comes up as well (untranslated in his stories*):
Iä! Iä! Cthulhu fhtagn! Ph'nglui mglw'nfah Cthulhu R'lyeh wgah'nagl fhtagn!
This phrase reappears in a lot of derivative Cthulhu works. For instance, the board game Arkham Horror has the text on the Cultist monster. There is even an Urban Dictionary definition that mentions this exact quote. And of course, there are memes:
He talked of his dreams in a strangely poetic fashion; making me see with terrible vividness the damp Cyclopean city of slimy green stone—whose geometry, he oddly said, was all wrong—and hear with frightened expectancy the ceaseless, half-mental calling from underground: “Cthulhu fhtagn”, “Cthulhu fhtagn”.
Note that while close, the words "Iä! Iä!" are not used here or anywhere else in the story.
The second part of the phrase does appear in the story, but isolated from "Cthulhu fhtagn"
What, in substance, both the Esquimau wizards and the Louisiana swamp-priests had chanted to their kindred idols was something very like this—the word-divisions being guessed at from traditional breaks in the phrase as chanted aloud:
“Ph’nglui mglw’nafh Cthulhu R’lyeh wgah’nagl fhtagn.”
So where does the phrase "Iä! Iä! Cthulhu fthagn!", and the longer version of the phrase, first appear?
Note, I'm looking for the exact phrase, not variations of the phrase or the phrase in fragments, although knowing about the evolution of the phrase would certainly be interesting supporting material.
* This phrase is in a language that Lovecraft made up. It's not translated in the stories, but the internet seems to translate it as "Hail! Hail! Cthulhu dreams! In his house at R'lyeh, dead Cthulhu waits dreaming!" I'm not sure where this translation originated.