“It is also a good rule not to put overmuch confidence in the observational results that are put forward until they are confirmed by theory.”

Ce n'est point l'observation mais la théorie qui m'a conduit à ce résultat que l'expérience a ensuite confirmé.
explaining how he was led to discover the law characterizing interference fringes, in [Œuvres complètes d'Augustin Fresnel, Imprimerie impériale, 1866, http://books.google.com/books?id=3QgAAAAAMAAJ, 61]

Pg. 67.
Against Method (1975)

As quoted in "Jamie Chung Reveals The 'Real' (Adorable) Reason She's Marrying Bryan Greenberg" in The Huffington Post (16 May 2014) https://www.huffpost.com/entry/jamie-chung-tj-maxx_n_5325129

The Notebooks of Leonardo da Vinci (1883), I Prolegomena and General Introduction to the Book on Painting

On Practice (1937)

Source: A Brief History of Time (1988), Ch. 1
Context: Any physical theory is always provisional, in the sense that it is only a hypothesis: you can never prove it. No matter how many times the results of experiments agree with some theory, you can never be sure that the next time the result will not contradict the theory. On the other hand, you can disprove a theory by finding even a single observation that disagrees with the predictions of the theory. As philosopher of science Karl Popper has emphasized, a good theory is characterized by the fact that it makes a number of predictions that could in principle be disproved or falsified by observation. Each time new experiments are observed to agree with the predictions the theory survives, and our confidence in it is increased; but if ever a new observation is found to disagree, we have to abandon or modify the theory.

Hung Hsiu-chu (2015) cited in " Cross-strait status quo is 'one China, same interpretation': KMT's Hung http://focustaiwan.tw/news/aipl/201505060036.aspx" on Focus Taiwan, 6 May 2015