Who coined the title, 'This is it'!?

I think its just a saying he has. in a letter I saw from a doctor to Michael, which I can't remember what it regarded and actually don't know if its real but believe it is, its from 2003 I think, it has a bunch of phrases Michael used such as, "You ain't seen nothing yet" and "This is it".
 
it seems like such a generic phrase, but it also seems that anything that MJ came up with, i heard it a lot after he said it. from 'i'm a lover, not a fighter', to 'this is it'. i just don't remember hearing the phrases before MJ coined them.
 
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