From Yuan Zhen's (
元稹, Tang dynasty)
离思五首·
其四, written mourning his late wife Wei Cong:
曾经沧海难为水,除却巫山不是云。取次花丛懒回顾,半缘修道半缘君 - 'Once you have seen the ocean, no other water counts; apart from the clouds of Mount Wu, no other clouds are clouds.' One of the most quoted laments in Chinese literature.