ko내가 공명을 얻은 것은 물고기가 물을 만난 것과 같다. 그러니 다시는 불평하지 말라.
ja私が孔明を得たのは、魚が水を得たようなものだ。だからもう二度と不平を言うな。
The scene behind the words
Liu Bei’s reply when Guan Yu and Zhang Fei bristled at his inseparability from the newly recruited Zhuge Liang. A lord casting himself as the fish and his minister as the water — the audacity of the image silenced the brothers. The original text of the idiom "fish and water."
Source: Records of the Three Kingdoms — Zhuge Liang