Which mathematician refused the Fields Medal?

The story behind the answer

Grigori Perelman is the only mathematician to refuse the Fields Medal (2006). He posted a proof of the Poincaré Conjecture using Ricci flow with surgery, building on Richard Hamilton’s ideas, and later declined both the Fields Medal and the $1 million Clay Millennium Prize, citing ethical concerns and discomfort with publicity and credit.

Context: The Fields Medal is awarded to mathematicians under 40; Terence Tao (2006) and Edward Witten (1990) accepted theirs. Andrew Wiles, who proved Fermat’s Last Theorem, was over 40 and thus ineligible. Memory tip: P for Poincaré, P for Perelman, and P for “Passed on the prize.”