From 《
资治通鉴·
唐玄宗开元二十四年》, describing how Chancellor Li Linfu would 'use sweet words to bait people, while secretly undermining them — never letting his words or expression betray him' (
好以甘言啗人,
而阴中伤之,
不露辞色). The phrase has stayed in use as a description of impressive self-control.