bīng​tàn​bù​yán​lěng​rè​zì​míng

sincerity is not expressed in words

冰炭不言,冷热自明 冰炭不言,冷熱自明 bīng​tàn​bù​yán​lěng​rè​zì​míng
sincerity is not expressed in words
ice and coal do not speak, yet it is obvious which is cold and which is hot; means that one's true nature or sincerity is apparent without needing to be stated