This is a well known stuff... :) ...but here again is the source (found via Marlene on Aidpage):

"Eight-year-old Virginia O'Hanlon wrote a letter to the editor of New York's Sun, and the quick response was printed as an unsigned editorial Sept. 21, 1897. The work of veteran newsman Francis Pharcellus Church has since become history's most reprinted newspaper editorial, appearing in part or whole in dozens of languages in books, movies, and other editorials, and on posters and stamps..."

Here are the letters...