Guan Yu lets Cao Cao escape at Huarong Pass

In Romance of the Three Kingdoms, after Cao Cao is defeated at the Battle of Chibi, Cao Cao manages to escape under the protection of Xu Huang, Zhang He, and Zhang Liao. Meanwhile, Guan Yu was sent by Zhuge Liang to guard Huarong Pass so he can wait and ambush Cao Cao's fleeing forces.When Cao Cao arrive near the ambush, Guan Yu and Guan Ping's forces appeared, blocking the pass. Cao Cao only had a few men and knew he would lose to Guan Yu's forces. Cao Cao's advisor, Cheng Yu whispered to his ear, telling him Guan Yu will let him go on account of their past dealings. So Cao Cao rode up to Guan Yu and bowed telling him to let him through this pass unharmed because he treated Guan Yu nice when he was under his service. Guan Yu finally let Cao Cao's forces pass through.

What happened historically?

 * This event is fictional.