„The law of the harvest is to reap more than you sow. Sow an act, and you reap a habit; sow a habit, and you reap a character; sow a character, and you reap a destiny.“
Reported in Phinneys' Calendar (1878), edited by Andrew Beers.
Related quotes

— Samuel Smiles Scottish author 1812 - 1904
Saying published anonymously in The Dayspring, Vol. 10 (1881) by the Unitarian Sunday-School Society, and quoted in Life and Labor (1887) by Smiles; this is most often attributed to George Dana Boardman, at least as early as 1884, but also sometimes attributed to William Makepeace Thackeray as early as 1891, probably because in in Life and Labor Smiles adds a quote by Thackeray right after this one, to Charles Reade in 1903, and to William James as early as 1906, because it appears in his Principles of Psychology (1890).
Misattributed
Source: Happy Homes and the Hearts That Make Them

— Charles Reade British writer 1814 - 1884
Possibly a misattribution, ascribed to Reade in Notes and Queries (9th Series) vol. 12, 17 October 1903. It appears (as an un-sourced quotation) in Life and Labor (1887) by Samuel Smiles and in the front of The Power of Womanhood by Ellice Hopkins (1899) htm http://www.gutenberg.org/files/13722/13722-h/13722-h..
Apparently a common saying in 19th century. It has been also attributed to an “old Chinese proverb”, Ralph Waldo Emerson (1803-1882), William Makepeace Thackeray (1811–1863), George Dana Boardman (1828-1903), Frances Elizabeth Caroline Willard (1839-1898), James Allen (1864-1912), Marcus Fabius Quintilianus http://en.wikipedia.org/wiki/Quintilian http://www.worldofquotes.com/author/Quintilian-(Marcus-Fabius-Quintilian)/1/index.html and William James.
No original source has ever been isolated. Its structure strongly reflects that of a ""classical Chinese"" set of aphorisms; and it may have been deliberately constructed in that form, by a non-Chinese, to imply an oriental (and, perhaps, far wiser) origin.
Finally, almost all of those who cite the complete piece:
::We sow a thought and reap an act;
::We sow an act and reap a habit;
::We sow a habit and reap a character;
::We sow a character and reap a destiny.
state that, in their view, it was written to expand an embellish the notion that was expressed at Proverbs XXIII:7 (""For as he thinketh in his heart, so is he"").
Attributed

— Stephen R. Covey, book The Seven Habits of Highly Effective People
Source: The 7 Habits of Highly Effective People

„As you sowed, so Scotland reaped.“
— Alex Salmond Scottish National Party politician and former First Minister of Scotland 1954
Scotland in the World Forum (February 4, 2008), Church of Scotland (May 25, 2009)

— Josefa Iloilo President of Fiji 1920 - 2011
Opening address to the National Day of Prayer in Suva, 15 May 2005 (excerpts) http://www.fiji.gov.fj/publish/page_4607.shtml

„4389. That, which you sow, you must reap.“
— Thomas Fuller (writer) British physician, preacher, and intellectual 1654 - 1734
Introductio ad prudentiam: Part II (1727), Gnomologia (1732)

„Everyone reaps what he sows.“
— Hasan al-Askari Eleventh of the Twelve Imams 846 - 874
Majlisi, Bihārul Anwār, vol.78, p. 338
General

„Who sows fear, reaps weapons.“
— Friedrich Dürrenmatt Swiss author and dramatist 1921 - 1990
About Tolerance (1977)

„Tupelo-o-o! Hey, Tupelo! You will reap just what you sow.“
— Nick Cave Australian musician 1957
Song lyrics, The Firstborn Is Dead (1985), Tupelo

„With tears we sow seeds of prayer in the earth of the heart, hoping to reap the harvest in joy.“
— Diadochos of Photiki Byzantine saint 400 - 467
§ 73
On Spiritual Knowledge and Discrimination (480 AD)

„5515. What's sowed in Youth, will be reaped in Age.“
— Thomas Fuller (writer) British physician, preacher, and intellectual 1654 - 1734
Introductio ad prudentiam: Part II (1727), Gnomologia (1732)

„They sowed the wind, and now they are going to reap the whirlwind.“
— Arthur Travers Harris Royal Air Force air marshal 1892 - 1984
Statement of 1942, at the start of the bombing campaign against Germany, as quoted in "Sir Arthur Harris & The Lancaster Bomber" at The British Postal Museum and Archive http://postalheritage.org.uk/page/ww2stamps-bomberharris
Context: The Nazis entered this war under the rather childish delusion that they were going to bomb everyone else, and nobody was going to bomb them. At Rotterdam, London, Warsaw, and half a hundred other places, they put their rather naive theory into operation. They sowed the wind, and now they are going to reap the whirlwind.

„As you sow in your subconscious mind, so shall you reap in your body and environment.“
— Joseph Murphy American writer 1898 - 1981
Source: The Power of Your Subconscious Mind -

„Don't come crying to me if your homes are attacked. You will reap what you sow.“
— Ian Paisley Politician and former church minister 1926 - 2014
After been forcibly carried out from the Assembly building by police(1986)http://news.bbc.co.uk/onthisday/hi/dates/stories/june/24/newsid_2519000/2519077.stm

— Frederick William Robertson British writer and theologian 1816 - 1853
Source: Dictionary of Burning Words of Brilliant Writers (1895), P. 211.
„She who sows vengeance must reap its bloody fruit.“
— Chitra Banerjee Divakaruni novelist, short story writer, poet, and essayist 1956
Source: The Palace of Illusions