The short answer
Quotes that allege to show this deliberately cut Darwin off right before he explains why any intuitive worries he or others might have about the evolution of the eye are unfounded.
The longer answer
Here are two typical examples of creationist attempts to find quotes from Darwin in support of the assertion.
Creationist Shmuel Waldman writes, “In fact, Darwin himself admitted that ‘The eye, to this day, gives me a cold shudder,’ being that [sic] it’s an ‘organ of extreme perfection'” (Waldman 2005:25).
Waldman’s citation for all of this is an 1860 letter from Darwin to Asa Gray (Waldman 2005:25, f4). Thanks to the Darwin Correspondence Project, it is a simple matter to find that letter, whereupon we discover that what Darwin really said was, “The eye to this day gives me a cold shudder, but when I think of the fine known gradations, my reason tells me I ought to conquer the cold shudder” (Darwin Correspondence Project Database 2011). To admit to a knee-jerk feeling that one knows is irrational hardly is to endorse that knee-jerk feeling.
Moreover, the phrase “organ of extreme perfection” does not occur anywhere in the letter. Waldman has cut out the part of the quote that overturns the case he wants to make, and has spliced in words from somewhere else, without citation.1 I will let you decide for yourself what this says about Waldman’s honesty and professionalism.
Creationist Scott Huse claims that “Charles Darwin acknowledged the utter inadequacy of the evolutionary theory when attempting to account for a structure such as the eye” (Huse 1983: 73). The evidence he offers for this claim is an alleged quote from Darwin:
To suppose that the eye, with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical, and chromatic aberration, could have been formed by natural selection, seems, I freely confess, absurd in the highest possible degree … The belief that an organ as perfect as the eye could have formed by natural selection is more than enough to stagger anyone. (Huse 1983:73)
That sounds pretty damning, but when we locate the source material—which happens to be On the Origin of Species—we get predictable results:
First, we discover that the ellipses (the “…”) cover nearly twenty pages of text.2
Second, Huse has deleted important material from each part. Here is what Darwin goes on to say immediately after “absurd in the highest possible degree”:
Yet reason tells me, that if numerous gradations of from a perfect and complex eye to one very imperfect and simple, each grade being useful to its possessor, can be shown to exist; if further, the eye does vary ever so slightly, and the variations be inherited, which is certainly the case; and if any variation or modification in the organ be ever useful to an animal under changing conditions of life, then the difficulty of believing that a perfect and complex eye could be formed by natural selection, though insuperable by our imagination, can hardly be considered real. (Darwin :207-208)
And here is the full sentence that Huse chops up after the ellipses:
Although the belief that an organ so perfect as the eye could have been formed by natural selection is more than enough to stagger anyone; yet in the case of any organ, if we know of a long series of gradations in complexity, each good for its possessor, then, under changing conditions of life, there is no logical impossibility in the acquirement of any conceivable degree of perfection through natural selection. (Darwin :226)
We see that Darwin’s stance is the exact opposite of what Huse says, and that Huse could not have failed to notice this unless he never actually looked at the source material. Again, I will let you draw your own conclusions about Huse’s integrity and professionalism.
Unfortunately, the kinds of omissions we see above are par for the course when creationists present allegedly damaging quotes from evolutionists.
1 Perhaps he copied it from one of the section headings in part six of On the Origin of Species, but without citation, it really is anyone’s guess where he saw it.
2 For reference, I am using the CRW 2004 edition of the Origin.
Darwin C. [2004.] The Origin of Species. New York: CRW.
Darwin Correspondence Project Database.  http://www.darwinproject.ac.uk/entry-2701 (letter no. 2701; accessed 7 Sep 2017).
Huse SM. 1983. The Collapse of Evolution. Grand Rapids, MI: Baker.
Waldman S. 2005. Beyond a Reasonable Doubt: Convincing Evidence of the Truths of Judaism. Jerusalem: Feldheim.