來而不往非禮也

From the Book of Rites, Book 1 (《禮記·曲禮上》):

太上貴德，其次務施報。禮尚往來。往而不來，非禮也；來而不往，亦非禮也。 [Classical Chinese, trad.]

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太上贵德，其次务施报。礼尚往来。往而不来，非礼也；来而不往，亦非礼也。 [Classical Chinese, simp.]

From: The Book of Rites, c. 4^th – 2^nd century BCE, translated based on James Legge's version

Tàishàng guì dé, qícì wù shī bào. Lǐ shàng wǎnglái. Wǎng ér bù lái, fēilǐ yě; lái ér bù wǎng, yì fēilǐ yě.

In the highest antiquity they prized (simply conferring) good; in the time next to this, giving and repaying was the thing attended to. And what the rules of propriety value is that reciprocity. If I give a gift and nothing comes in return, that is contrary to propriety; if the thing comes to me, and I give nothing in return, that also is contrary to propriety.

來而不往非禮也

1. 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.