[math-fun] Isaac Newton quote (?)

I could've sworn that Isaac Newton, when asked how he got his solutions to math and physics problems, replied "By thinking upon them constantly", but I've tried entering this sentence (and semantically-equivalent variants) into Google, and I haven't been able to find it. Does anyone know the quote I'm mis-remembering? I should say I've done my due diligence (five minutes with a search engine) before daring to bother you all. Jim