来而不往非礼也

it is impolite not to reciprocate

lái ér bù wǎng fēi lǐ yě ㄌㄞˊ ㄦˊ ㄅㄨˋ ㄨㄤˇ ㄈㄟ ㄌㄧˇ ㄧㄜˇ