1787 March 21. (to Martha Jefferson Randolph). "Of all the cankers of human happiness, none corrodes it with so silent, yet so baneful a tooth, as indolence. Body and mind both unemployed, our being becomes a burthen and every object about us loathsome, even the dearest, Idleness begets ennui, ennui the hypochondria, and that a diseased body. No laborious person was ever hysterical. Exercise and application produce order in our affairs, health of body, chearfulness of mind, and these make us precious to our friends. It is while we are young that the habit of industry is formed. If not then, it never is afterwards. The fortune of our lives therefore depends on employing well the short period of youth. If at any moment, my dear, you catch yourself in idleness, start from it as you would from a precipice of a gulph. You are not to consider yourself unemployed while taking exercise."

1787 May 21. (to Martha Jefferson Randolph). "A mind always employed is always happy...The idle are the only wretched. In a world which furnishes so many emploiments which are useful, and so many which are amusing, it is our own fault if we we ever know what ennui is, or if we are ever driven to the miserable resource of gaming, which corrupts our disposition, and teaches us a habit of hostility against all mankind."

1790 April 26. (to Martha Jefferson Randolph). "I think both you and Mr. Randolph will suffer ennui at Richmond. Interesting occupations are essential to happiness: indeed the whole art of being happy consists in the art of finding emploiment. I know none so interesting, and which croud upon us so much, as those of a domestic nature."

Source: http://www.monticello.org/site/jefferson/quotations-idleness