“Any act often repeated soon forms a habit : and habit allowed, steadily gains in strength.”

Vol. I, ch. 10
Lady Holland's Memoir (1855)

1951 - 1968, Apropos of Ready Mades', 1961

The Westminster Review, vol. 6 (1826), p. 13
Context: This habit of forming opinions, and acting upon them without evidence, is one of the most immoral habits of the mind.... As our opinions are the fathers of our actions, to be indifferent about the evidence of our opinions is to be indifferent about the consequences of our actions. But the consequences of our actions are the good and evil of our fellow-creatures. The habit of the neglect of evidence, therefore, is the habit of disregarding the good and evil of our fellow-creatures.

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

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

An Old Man's Thoughts on Many Things, Of Education I

Golden Sayings of Epictetus
Context: If you have given way to anger, be sure that over and above the evil involved therein, you have strengthened the habit, and added fuel to the fire. If overcome by a temptation of the flesh, do not reckon it a single defeat, but that you have also strengthened your dissolute habits. Habits and faculties are necessarily affected by the corresponding acts... One who has had fever, even when it has left him, is not in the same condition of health as before, unless indeed his cure is complete. Something of the same sort is true also of diseases of the mind. Behind, there remains a legacy of traces and of blisters: and unless these are effectually erased, subsequent blows on the same spot will produce no longer mere blisters, but sores. If you do not wish to be prone to anger, do not feed the habit; give it nothing which may tend to its increase. At first, keep quiet and count the days when you were not angry: 'I used to be angry every day, then every other day: next every two, next every three days!' and if you succeed in passing thirty days, sacrifice to the Gods in thanksgiving. (75).