“The art of discovery is confused with the logic of proof and an artificial simplification of the deeper movements of thought results. We forget that we invent by intuition though we prove by logic.”

Internet Encyclopedia of Philosophy
Context: The presentation of facts in logical form contributes to a confusion between discovery and proof. If the process of discovery were mere synthesis, any mechanical manipulator of prior partial concepts would have reached the insight and it would not have taken a genius to arrive at it.

C'est par la logique qu'on démontre, c'est par l'intuition qu'on invente.
Part II. Ch. 2 : Mathematical Definitions and Education, p. 129
Science and Method (1908)

From his "Autobiographische Skizze" (18 April 1955), original German version here http://philoscience.unibe.ch/documents/kursarchiv/WS99/Skizze.pdf. Translation from Subtle is the Lord: The Science and the Life of Albert Einstein by Abraham Pais (1982), p. 131 http://books.google.com/books?id=U2mO4nUunuwC&lpg=PP1&pg=PA131#v=onepage&q&f=false. Pais notes that when he said "during that year", he was referring to some time between October 1895 and early fall 1896.
Variant: "Innovation is not the product of logical thought, even though the final product is tied to a logical structure."
Original German version: Während dieses Jahres in Aarau kam mir die Frage: Wenn man einer Lichtwelle mit Lichtgeschwindigkeit nachläuft, so würde man ein zeitunabhängiges Wellenfeld vor sich haben. So etwas scheint es aber doch nicht zu geben! Dies war das erste kindliche Gedanken-Experiment, das mit der speziellen Relativitätstheorie zu tun hat. Das Erfinden ist kein Werk des logischen Denkens, wenn auch das Endprodukt an die logische Gestalt gebunden ist. ("Autobiographische Skizze", p. 10)
1950s
Context: During that year in Aarau the question came to me: If one runs after a light wave with [a velocity equal to the] light velocity, then one would encounter a time-independent wavefield. However, something like that does not seem to exist! This was the first juvenile thought experiment which has to do with the special theory of relativity. Invention is not the product of logical thought, even though the final product is tied to a logical structure.

Source: The Structure of Scientific Revolutions (1962), VI. Anomaly and the Emergence of Scientific Discoveries, p. 52 (2012 ed.)

Ahmadinejad on His Proposal for a Jewish State in Europe and on Iran's Nuclear Energy http://www.memritv.org/Transcript.asp?P1=987 Jan. 2006.
2006

Source: The Heritage Universe, Summertide (1990), Chapter 13, “Summertide Minus Ten” (p. 150)

Source: Urban renewal and social conflict in Paris, 1972, p. 93

Source: The Design of Everyday Things (1988, 2002), Ch. 5, pp. 114–115.

Gottlob Frege in: Dagobert David Runes (1962). Readings in epistemology, theory of knowledge and dialectics. p. 334