Where does the theory of "son-in-law of life and death" come from?

Life and death are rich, and the theory of Zi Cheng comes from The Book of Songs and "Guo Feng's peak drumming". Full text drums boring, active use of troops. Cao Tuguocheng, I travel south alone. From Sun Zizhong, Chen Pinghe and Song Dynasty. No, I'm going home. I'm worried. The man who lives in the same place lost his horse. Look for it, under the forest. Life and death are generous, and Zi Cheng says. Hold your hand and grow old with your son. I am rich, but I am still alive. Sorry, but I believe it.

Drumming is a typical war poem in The Book of Songs, the first poetry collection in ancient China. This is a homesick song sung by a soldier who has gone to a foreign country and can't go home for a long time. The whole poem consists of five chapters, each with four sentences. In the first three chapters, people are asked to tell the story of the expedition, which is dense and full of complaints. The last two chapters describe the mutual encouragement between soldiers, which is touching. This poem describes the sorrow of foot soldiers who have been fighting for a long time. Among them, the description of soldiers' feelings is "life and death" and "zi cheng" "Hold your hand and grow old with your son" is also used to describe the deep friendship between husband and wife in later generations.