“Quality is not an act, it is a habit.”

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

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).

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 Essay on Charity, and Charity-Schools", p. 319
The Fable of the Bees (1714)

Misattributed
Variant: We are what we repeatedly do, therefore excellence is not an act, but a habit.
Source: Will Durant, The Story of Philosophy: The Lives and Opinions of the World's Greatest Philosophers (1926), reprinted in Simon & Schuster/Pocket Books, 1991, ISBN 0-671-73916-6], Ch. II: Aristotle and Greek Science; part VII: Ethics and the Nature of Happiness: "Excellence is an art won by training and habituation: we do not act rightly because we have virtue or excellence, but we rather have these because we have acted rightly; 'these virtues are formed in man by his doing the actions'; we are what we repeatedly do. Excellence, then, is not an act but a habit: 'the good of man is a working of the soul in the way of excellence in a complete life... for as it is not one swallow or one fine day that makes a spring, so it is not one day or a short time that makes a man blessed and happy'" (p. 76). The quoted phrases within the quotation are from the Nicomachean Ethics, Book II, 4; Book I, 7. The misattribution is from taking Durant's summation of Aristotle's ideas as being the words of Aristotle himself.

Preface
1900s, Major Barbara (1905)

Variant: We are what we repeatedly do. Greatness then, is not an act, but a habit
Source: The Story of Philosophy (1926), p. 87. The quoted phrases within the quotation are from the Nicomachean Ethics, Book II, 4; Book I, 7.
Context: Excellence is an art won by training and habituation: we do not act rightly because we have virtue or excellence, but we rather have these because we have acted rightly; 'these virtues are formed in man by his doing the actions'; we are what we repeatedly do. Excellence, then, is not an act but a habit: 'the good of man is a working of the soul in the way of excellence in a complete life... for as it is not one swallow or one fine day that makes a spring, so it is not one day or a short time that makes a man blessed and happy'.

BBC obituary (2004)