If you view Aristotle's "Metaphysics" at http://classics.mit.edu/Aristotle/metaphysics.mb.txt and search for the word "know" (including words with "know" n them), you will see that he strongly suggests it several times, though not as a direct quote, at least not in this translation. Perhaps the most relevant paragraph:
"Some of the sensible substances are generally admitted to be
substances, so that we must look first among these. For it is an
advantage to advance to that which is more knowable. For learning
proceeds for all in this way-through that which is less knowable by
nature to that which is more knowable; and just as in conduct our task
is to start from what is good for each and make what is without
qualification good good for each, so it is our task to start from what
is more knowable to oneself and make what is knowable by nature
knowable to oneself. Now what is knowable and primary for particular
sets of people is often knowable to a very small extent, and has
little or nothing of reality. But yet one must start from that which
is barely knowable but knowable to oneself, and try to know what is
knowable without qualification, passing, as has been said, by way of
those very things which one does know."
The point he's making is the opposite of what the quotation suggests: Aristotle believes that all things are knowable, but, in a specific individual's journey towards complete knowledge, there will be times that new knowledge shows there are more things to learn.
The last instance of 'know' occurs in a somewhat weak sentence "But evidently in a sense knowledge is universal, and in a sense it is not."