來而不往非禮也 [来－－－－礼－]

Jyutping loi4 ji4 bat1 wong5 fei1 lai5 jaa5
Pinyin lái ér bù wǎng fēi lǐ yě

Definitions (CC-CEDICT)
  1. not to reciprocate is against etiquette (classical)
  2. to respond in kind
Definitions (粵典–words.hk)
  1. x
Definitions (Wiktionary)
  1. proverb
    It is impolite not to reciprocate; Friendship cannot stand always on one side; It's impolite not to make a return for what one receives; One should return as good as one receives.