Tao Te Ching · Ch. 69

用兵有言:吾不敢為主,而為客;不敢進寸,而退尺。是謂行無行;攘無臂;扔無敵;執無兵。禍莫大於輕敵,輕敵幾喪吾寶。故抗兵相若,哀者勝矣。

Yòng bīng yǒu yán: Wú bù gǎn wéi zhǔ, ér wéi kè; Bù gǎn jìn cùn, ér tuì chǐ. Shì wèi xíng wú xíng; Rǎng wú bì; Rēng wú dí; Zhí wú bīng. Huò mò dà yú qīng dí, qīng dí jī sāng wú bǎo. Gù kàng bīng xiāng ruò, āi zhě shèng yǐ.

There is a saying among those who use troops: I dare not act as the host but act as the guest; I dare not advance an inch but rather retreat a foot. This is called marching without marching; rolling up the sleeves without showing the arms; advancing without a weapon in hand; taking hold without an opponent. There is no greater calamity than underestimating an opponent; to underestimate an opponent is to nearly lose my treasure. Therefore when opposing forces are equal, the one who grieves will win.

— Tao Te Ching 69

Source: Wang Bi edition (Chinese) + Hanyu Pinyin transliteration