Stevenson's admission of the earlier stories and authors he'd
plagiarised borrowed ideas from comes in My First Book - his little-known preface to Treasure Island, first published in McClure's Magazine in September 1894.
First Stevenson acknowledges very readily some minor ideas and motifs taken from other writers:
It is not to be wondered at, for stolen waters are proverbially sweet. I am now upon a painful chapter. No doubt the parrot once belonged to Robinson Crusoe. No doubt the skeleton is conveyed from Poe.
... but goes on to say that these small acts of 'plagiarism' are unimportant and that his greatest debt is to Washington Irving, from whom he took almost wholesale the character of Billy Bones. (I won't go into this in too much detail since it's not the main subject of the question here; if you're interested in more evidence for the Irving connection, you can read My First Book in full here and also see very similar descriptive passages from Treasure Island and Irving's Tales of a Traveller reproduced together in Appendix C of this edition of Treasure Island.) Later on, he returns to Poe by saying:
The map was the chief part of my plot. For instance, I had called an islet
Skeleton Island, not knowing what I meant, seeking only for the immediate picturesque; and it was to justify this name that I broke into the gallery of Mr Poe and stole Flint's pointer.
Both of these nods to Poe must refer specifically to "The Gold-Bug", in which the famous real-world pirate Captain Kidd left signposts to his fictional treasure which included a skull and two skeletons, presumably belonging to some of his former crewmen.
How I found this: after some failures at looking for answers on the internet using various different search terms, I finally discovered here a direct quote from Stevenson, which I could then paste into Google to find its source.