In chapter 45 of Romance of the Three Kingdoms, Jiang Gan tells Cao Cao: “Since childhood I have been Zhou Yu’s schoolmate and close friend.” He offers to talk Zhou Yu into surrendering.
Zhou Yu welcomes him, drinks with him late into the night and shares his bed. Jiang Gan finds papers on the desk, among them a letter from Cao Cao’s admirals, Cai Mao and Zhang Yun, and reads it in secret. Back in camp, he hands it to Cao Cao, who has both admirals beheaded. Then Cao Cao realizes: “I have fallen for a trick.”
An older account, quoted in a commentary to the Records of the Three Kingdoms, tells it differently. There, Jiang Gan says that he and Zhou Yu come from the same region. After a meal, Zhou Yu sends him to his lodgings: “I have a secret matter just now.” Three days later, Zhou Yu shows him the camp and its stores. Jiang Gan only laughs and in the end says nothing.