As far as I can tell, ~100% of the (many) references to that quote can be traced to Richard Hamming's 1986 lecture, "You And Your Research" (http://www.cs.virginia.edu/~robins/YouAndYourResearch.html), which includes that quote and attributes it to Newton (but without a specific citation). I'd be curious where Hamming got it from, since I'm not able to find either that exact quote or anything similar quoted by anybody pre-1986 (even as an apocryphal/uncited quote), at least from some brief Google Books and Lexis-Nexis digging.