After tapping a few words into my much loved google toolbar (I love Opera), I was told that the first use of scratching as a musical element - Although the DJ Grand Wizard Theodore is credit with it's invention - was first recorded on `Adventures of Grandmaster Flash on the Wheels of Steel', released in 1981. I've seen this documentary, "Scratch", and most of the DJs interviewed continuously go on about a Herbie Hancock song, "Rockit", which I first thought to be the first instance of scratching as a musical element. But it was released 5 years after the Grandmaster Flash recording.
I'm using http://www.mercurynews.com/mld/mercurynews/entertainment/music/7162946.htm as a resource