Scribbled note in the margins of his copy of Diophantus's Arithmetica. He did not live to provide the promised proof, and the conjecture became famous as Fermat's Last Theorem. In 1993 Andrew Wiles, a British mathematician, claimed to have discovered the proof.
"The Feynman Lectures on Physics (volume I; lecture 3: "The Relation of Physics to Other Sciences")". Book by Richard P. Feynman, Robert B. Leighton and Matthew Sands, 1964.