I think that Tolkien mentioned it in "On Fairy Stories" and Tom Shippey expanded on it in "The Road to Middle-Earth". Forgive me for simplifying because I'm eating and can't spare a hand to search. :D Tolkien, a real philologist, that is "lover of language", thought that a word or a name could not exist by itself, it had to refer back to something. Now, "elf, alf, elb, alb", are words found in lots of different traditions and peoples. The word appears in names such as "Aelfwine", which Tolkien used in his very interesting unpublished draft of a novel, "The Lost Road": an old Germanic name which means "elf-friend" (that is, "Elendil"!)

What was an "elf" in all those ancient traditions? The details vary: some were "dark", and some were benign spirits, often skilled in making beautiful things, and sometimes confused with goblins and dwarves. But they were mostly very powerful beings, that's why Tolkien objected so much to Shakespeare's fairies.

So he thought that if there were so many references to them, something of that kind must have existed once. He never really made a theory out of this: it remained a half-spoken dream. But certainly he "vindicated" his concept of what Elves should be with characters such as Feanor and Galadriel.