有志者事竟成

From the Book of the Later Han (《后汉书·耿弇传》): when general Geng Yan (耿弇) successfully completed a campaign Emperor Guangwu had once thought impossible, the emperor praised him with 有志者事竟成 — 'one who has determination will, in the end, succeed.' The phrase has been quoted as motivational shorthand ever since, the Chinese equivalent of 'where there's a will, there's a way.'

Classical structure: 有志者 (one who has resolve) + 事 (the matter) + 竟 (in the end, finally) + 成 (succeed). 竟 here means 'eventually,' not 'unexpectedly' as in modern Chinese.