“The harder I work, the luckier I get'. It was Thomas Jefferson who started the stream of variations on that theme. He should have added, 'The harder I work on one thing, the unluckier I get on all the other commitments I haven’t had time for'.”

Originated with Samuel Goldwyn as a paraphrase of a proverb from a collection by Coleman Cox, but similar proverbs have existed since the 16th century. http://quoteinvestigator.com/2012/07/21/luck-hard-work/
Misattributed

Source: Disrupt You! (2015), p.151

It was Thomas Jefferson who started the stream of variations on that theme. He should have added, 'The harder I work on one thing, the unluckier I get on all the other commitments I haven’t had time for'.
Lean Logic, (2016), p. 472, entry on Time Fallacies http://www.flemingpolicycentre.org.uk/lean-logic-surviving-the-future/

Deathbed assertion, as quoted in Outlook Business, Vol. 3, No. 4 (23 February 2008)

2001 - 2010, Out to Lunch with Isa Genzken' (2009)

Source: Disgrace

Source: Anybody Out There?

quote in his letter to brother Theo, from The Hague, The Netherlands in Jan. 1882; as quoted in Vincent van Gogh, Alfred H. Barr; Museum of Modern Art, New York, 1935 https://www.moma.org/documents/moma_catalogue_1996_300061887.pdf, p. 20 (letter 171)
1880s, 1882

Has been attributed to Stephen Leacock's "Literary Lapses" (1910), but the quote does not appear in the Project Gutenberg edition http://www.gutenberg.org/files/6340/6340.txt of this work.
Misattributed
Variant: I find that the harder I work, the more luck I seem to have.
Variant: I'm a great believer in luck, and I find the harder I work the more I have of it.