In chapter 18 of the Water Margin, in its 100-chapter version, the county clerk Song Jiang learns that his friend Chao Gai is to be arrested. He tells the arresting officer that this official business “must not be leaked lightly to anyone.” Then he leaves the officer waiting in a teahouse and gallops off to warn Chao Gai. “He and I are bound heart and belly,” Chao Gai tells the others. “He is my sworn brother.”
In chapter 20, Chao Gai, now the leader at Liangshan, sends Liu Tang with a letter and a hundred taels of gold, to thank Song Jiang for saving their lives. Song Jiang keeps the letter and takes one bar of the gold.