King Cheng of Zhou was playing with his younger brother, Shu Yu. He cut a paulownia leaf into the shape of a jade tablet, gave it to him and said: “With this I give you a fief.” The court historian Yi asked him to choose a day for the ceremony. “I was only joking,” said the king.
The historian answered: 天子无戏言. “The Son of Heaven does not jest. When he speaks, the historians record it, the rites complete it and music sings it.” So Shu Yu was given the state of Tang. (Sima Qian, Records of the Grand Historian)