There is only one eight-character idiom whose first character is mouth:
The mouth has no choice of words, the body has no choice of actions
kǒu
wú
zé
yán, shēn
wú
zé
xíng
Explain that words and deeds are in line with etiquette and law, and there is nothing to criticize
Source: "The Classic of Filial Piety·Qing Dafu": "That is why one should not speak out against the law, do not follow the Tao, one cannot choose what to say with one's mouth, and one cannot choose one's deeds with one's body.
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Structured compound sentence idioms
Used as object and attributive; used in written language
Synonyms spoken without choice