When drinking water, one should not forget its source. During the Southern and Northern Dynasties, Yu Xin (513-581) was forced to leave home and stay away from his home state for more than 20 years. Often homesick, he wrote in one of his essays that he always thought of the source of the water whenever he drank form the river, expressing his longing for his hometown in the Southern Dynasties. Eventually this became an idiom: when drinking water one should think of its source, implying that one should never forget his origin and always be grateful. This virtue has always been highly praised.

When eating fruits I think of the fruit trees; when drinking water I think  of the source of the river.  (Yu Xin: The Tune of Zhi)