Earlier quoted context omitted.
Bizarre - the last link I was on was a random ESPN quiz with UVa star Joe Harris in which that quote was one of the questions. (I realize this is a pretty meaningless and useless comment, but seeing this quote 2 times in less than 10 seconds was too much not to mention).
I'm unable to find a definitive reference, but Gary Player (golfing legend) is also attributed with the quote: "The harder I practice, the luckier I get." Perhaps that's why it was on ESPN. Pretty sure George Simmons has this quote of Player's on one of the chapter pages in his "Precalculus Mathematics in a Nutshell" book.
http://quoteinvestigator.com/2010/07/14/luck/
The upshot is that its origin is unknown, but goes back at least to a Walter Winchell column from 1949. Gary Player used it and became associated with it. The attribution to Jefferson is nonsense, of course.