The chains of habit are too light to be felt until they’re too heavy to be broken.

Samuel Johnson (1709-1784) English writer, lexicographer, critic

Story (1748-04), “The Vision of Theodore, The Hermit of Teneriffe, Found in His Cell,” The Gentleman’s Magazine, Vol. 18 [paraphrase]

This phrase is frequently used by American investor Warren Buffett, but is most commonly credited to Samuel Johnson, always without citation because it is not Johnson's actual words. The actual passage from Johnson's moral allegory reads:

It was the peculiar artifice of Habit not to suffer her power to be felt at first. Those whom she led, she had the address of appearing only to attend, but was continually doubling her chains upon her companions; which were so slender in themselves, and so silently fastened, that while the attention was engaged by other objects, they were not easily perceived. Each link grew tighter as it had been longer worn, and when, by continual additions, they became so heavy as to be felt, they were very frequently too strong to be broken.

In Maria Edgeworth, Tales For Young People, "Forester" (1806), she includes this passage:

"I thought the lesson I got at the brewery," said he, "would have cured me for ever of this foolish trick; but the diminutive chains of habit, as somebody says, are scarecely ever heavy enough to be felt, till they are too strong to be broken."

Edgeworth footnotes after "diminutive chains of habit" that this comes from "Dr. Johnson's Vision of Theodore."

While "Vision of Theodore" has a couple of references to "diminutive" and a number of allegorical passages to "chains of Habit," it took a closer reading to find the passage above, replicated below with key words bolded:

It was the peculiar artifice of Habit not to suffer her power to be felt at first. Those whom she led, she had the address of appearing only to attend, but was continually doubling her chains upon her companions; which were so slender in themselves, and so silently fastened, that while the attention was engaged by other objects, they were not easily perceived. Each link grew tighter as it had been longer worn, and when, by continual additions, they became so heavy as to be felt, they were very frequently too strong to be broken.

It appears that later 19th and 20th Century variants on the quote paraphrase stem from Edgeworth's, but attributed to Johnson because of her footnote. This sort of fracturing of a quotation after a heavy paraphrase is not unusual. These variants include, in addition to the one noted above:

• "The diminutive chains of habit are seldom heavy enough to be felt till they are too strong to be broken." [1856]
• "The diminutive chains of habit are scarcely ever heavy enough to be felt, till they are too strong to be broken." [1859]
• "The diminutive chains of habit are generally too small to be felt till they are too strong to be broken." [1861]
• "The chains of habit are too weak to be felt until they are too strong to be broken." [1903]

The phrase "diminutive chains of habit" appears to predate Edgeworth, e.g., 1798.

For more information on this quotation's origin (which I had figured out before I found that the work had already been done on it) see Quote Origin: The Chains of Habit Are Too Light To Be Felt Until They Are Too Heavy To Be Broken – Quote Investigator®.