Introduction to "Guan Yu's Letter to Prime Minister Cao": It is said that when Guan Yu left Xuchang, he left a letter to Prime Minister Cao. The letter wrote: "The sun is in the sky, and the heart is in the human body. The sun is in the sky. It shines in all directions; the heart is in people to show their sincerity. The person who is sincere is also faithful. Someone once said: When the master dies, he will be assisted; if the master survives, he will return. Lord Liu's kindness. The kindness of Lord Liu has been repaid, and the righteousness has not been broken. Now the Lord's loss is known, and I look for the traces of the merits of the white horse. On the south slope, the Prime Minister's kindness was fully repaid. Every gift he left was sealed in the treasury. He looked down at the tower and looked down to learn from it."