Great works are performed not by strength, but perseverance; yonder palace was raised by single stones, yet you see its height and spaciousness. He that shall walk with vigour three hours a day, will pass in seven years a space equal to the circumference of the globe.
 Samuel Johnson, The History of Rasselas, Prince of Abissinia (1759). copy citation

Context

“The Prince for a moment appeared to be discouraged. “ Sir, ” said his companion, “ practice will enable us to continue our labour for a longer time. Mark, however, how far we have advanced, and ye will find that our toil will some time have an end. Great works are performed not by strength, but perseverance; yonder palace was raised by single stones, yet you see its height and spaciousness. He that shall walk with vigour three hours a day, will pass in seven years a space equal to the circumference of the globe. This Rasselas considered as a good omen. “ Do not disturb your mind, ” said Imlac, “ with other hopes or fears than reason may suggest; if you are pleased with the prognostics of good, you will be terrified likewise with tokens of evil, and your whole life will be a prey to superstition.” source