Gao Gan
In the following year, when Cao Cao left Ji Province to attack Yuan Shao's nephew Gao Gan in Bing Province, he left his son Cao Pi in charge of Ye, the capital of Ji Province, and instructed Cui Yan to mentor Cao Pi. Once, when Cao Pi went on a hunting excursion to enjoy himself, Cui Yan wrote a long letter to him, haranguing the former on overly indulging in personal pleasures and neglecting his duties. Cao Pi later sent a reply, stating that he had destroyed his hunting equipment and thanking Cui Yan for his lecture.