“I have peasant origins, this kneading hardness. One grandfather was a peasant, the other a foreman in a cigarette factory. I trained my mind whole life. For example, I studied poems by heart, from Mickiewicz to Mayakovsky. (in answer to the question of how he managed to stay active scientifically for so long)”

in answer to the question of how he managed to stay active scientifically for so long
Kobos, Andrzej (2009). Po drogach uczonych (in Polish). 4. Kraków: Polska Akademia Umiejętności, pp. 383–398. ISBN 978-83-7676-021-6.

As quoted in "Louis Pasteur" in The Catholic Encyclopedia (1913)
As quoted in Letter to an Atheist (2007) by Michael Patrick Leahy, p. 61
His descendents, Louis-Pasteur Vallery-Radot, and Maurice Vallery-Radot disputed the authenticity of such statements. According to Maurice Vallery-Radot, Pasteur (1994), p. 378, the attributed assertion first appeared in the Semaine religieuse .... du diocèse de Versailles (6 October 1895), p. 153, shortly after the death of Pasteur.
Disputed
Variant: I have the faith of a Breton peasant and by the time I die I hope to have the faith of a Breton peasant's wife.

Source: Lord Malquist and Mr Moon (1966), Ch. 2: A Couple of Deaths and Exits.

Source: Aphorisms and Reflections (1901), p. 234

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Quote in his letter to brother Theo from Nuenen, The Netherlands, Summer 1885; as quoted in Vincent van Gogh, edited by Alfred H. Barr; Museum of Modern Art, New York, 1935 https://www.moma.org/documents/moma_catalogue_1996_300061887.pdf, (letter 400) p. 21
1880s, 1885

From the 1997 television program Stephen Hawking's Universe http://www.pbs.org/wnet/hawking/html/home.html
Unsourced variant: All of my life, I have been fascinated by the big questions that face us, and have tried to find scientific answers to them. Perhaps that is why I have sold more books on physics than Madonna has on sex. This quote seems to combine the above sentence from Stephen Hawking's Universe with a statement from the Foreword to The Illustrated Brief History of Time: As Nathan Myhrvold of Microsoft (a former post-doc of mine) remarked: I have sold more books on physics than Madonna has on sex.

Ayittey, p. 109

Source: Hell's Angels: A Strange and Terrible Saga