Leading cadres should be 'quick in action, cautious in speech.'
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From 《论语·学而》 (Analects, book 1): 子曰：君子食无求饱，居无求安，敏于事而慎于言，就有道而正焉，可谓好学也已 (The Master said: The gentleman in eating seeks no full belly, in dwelling no comfort; quick in action yet cautious in speech; drawing near to those with the Way to correct himself - such a one may be called fond of learning). A favored Xi Jinping quotation on official conduct.
usage
于 is the classical preposition for 'at / in regard to' - 敏于事 = 'quick with respect to affairs.' A common variant is 讷于言而敏于行 (slow in speech, quick in action, from 《论语·里仁》). Same sense, different chapter.
