His idol refused to hire him the first time he applied.
Buffett originally wanted to work with his idol, and author of "The Intelligent Investor," Benjamin Graham, but Graham rejected him because he wasn't Jewish (Graham was saving a spot at his firm for someone Jewish, since at the time Jewish people had a tougher time landing work on Wall Street).
Buffett wouldn't take no for an answer, and continued pitching Graham ideas until he eventually hired him.