Genius is one percent inspiration, ninety-nine percent perspiration
Genius is largely the result of hard work, rather than an inspired flash of insight.
Few proverbial sayings can be attributed to a named individual. In this case, we can name the coiner of the expression - the American inventor Thomas Alva Edison.
Edison is first reported as saying "Genius is one per cent inspiration, ninety-nine per cent perspiration" sometime around 1902, in the September 1932 edition of Harper's Monthly Magazine.
It is impossible to know at this remove whether the Harper's journalists were accurate in their attribution, although no one else appears to have uttered the words before they reported them in 1932.
Edison may have come up with the neatest line but several others had expressed very similar thoughts before 1902. The Comte de Buffon (a.k.a. George-Louis Leclerc), 1707–88, was attributed with this line in Hérault de Séchelles' Voyage à Montbar, in 1803:
Genius is only a greater aptitude for patience.
Thomas Carlyle is widely reported to have coined the term "Genius is an infinite capacity for taking pains". What he actually said, in the History of Frederick the Great, written 1858–65, was:
'Genius' (which means transcendent capacity of taking trouble, first of all).
Not to be outdone, that disciple of earnest endeavour John Ruskin wasn't to be left out. In Notes by Mr Ruskin on His Collection of Drawings by the late J. M. W. Turner, 1878, he made the observation that:
I know of no genius but the genius of hard work.
See also: the List of Proverbs.