《汉书·传·武五子传》
正在这时皇帝派专使来赐给燕王玺书，上面写道： 当初高祖皇帝取得天下，封立子弟为王用以藩屏江山社稷。
会天子使使者赐燕王玺书曰： 昔高皇帝王天下，建立子弟以藩屏社稷。
Just then, a special envoy from the emperor arrived and gave King Yan a sealed imperial decree, which stated: "When Emperor Gaozu first established his dynasty, he enfeoffed his sons and younger brothers as kings to protect the empire and the ancestral temples.