冰炭不言，冷熱自明 [－－－－－－热－－]

Jyutping bing1 taan3 bat1 jin4 , laang5 jit6 zi6 ming4
Pinyin bīng tàn bù yán , lěng rè zì míng

Definitions (CC-CEDICT)
  1. ice or coals, whether hot or cold goes without saying (idiom); fig. sincerity is not expressed in words