兄弟鬩牆

From the Classic of Poetry, poem 164 (《詩經·小雅·常棣》):

兄弟鬩于牆，外禦其務。
每有良朋，蒸也無戎。 [Pre-Classical Chinese, trad.]

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兄弟阋于墙，外御其务。
每有良朋，蒸也无戎。 [Pre-Classical Chinese, simp.]

From: The Classic of Poetry, c. 11^th – 7^th centuries BCE, translated based on James Legge's version

Xiōngdì xì yú qiáng, wài yù qí wǔ.
Měi yǒu liángpéng, zhēng yě wú róng. [Pinyin]

Brothers may quarrel inside the walls,
But they will oppose insult from without.
When friends, however good they may be,
Will not afford help.

兄弟鬩牆

1. (figurative) internal dispute