"First Monday Interview with Linus Torvalds: What motivates free software developers?". Interview with Rishab Aiyer Ghosh, firstmonday.org. March 2, 1998.
Letter to Duc de Richelieu, 18 June 1744. Although this saying is now associated with Voltaire, he is obviously quoting an Italian proverb here. The French form, which he used later, is Le mieux est l'ennemi du bien.