The earliest well known example is fictional, it occurs in the 1902 Sherlock Holmes novel 'The Hound of the Baskervilles.'

I couldn't find any reference to any earlier examples either fictional or non-fictional. It makes sense that the trope would be lifted from Sherlock Holmes because so much else that we associate with crime and detective fiction is as they were genre defining and very popular.

http://www.nytimes.com/2011/07/03/movies/ransom-note-age-old-device-with-dramatic-payoff.html?_r=0

According to A. Stern 2012 ("The Ransom Note Effect : Eclectic Typography in American Visual Culture" http://sunzi.lib.hku.hk/ER/detail/hkul/5121782) Baskervilles was indeed the progenitor of the "cut-out note" trope (though in that story it appears as a kind of warning, rather than a ransom demand per se). Stern states that there was no apparent historical or literary precedent (i.e. Doyle wholly invented it for the story).

More info at: http://tvtropes.org/pmwiki/pmwiki.php/Main/CutAndPasteNote