《资治通鉴·隋纪·隋纪三》
牛弘的回答不合文帝的意思，于是文帝脸色一变，对东宫的官吏僚属说： 仁寿宫离这里不远，但是我每次返回京师都得严格准备仪仗保卫，就象进入敌国一样。
弘对既失旨，上因作色，谓东宫官属曰： 仁寿宫此去不远，而令我每还京师，严备仗卫，如入敌国。
Niu Hong’s answer did not conform to Emperor Wen’s intentions, so Emperor Wen’s face changed. He said to the officials of the crown prince’s palace: “Renshou Palace is not far from here, but every time I return to the capital city, I need to prepare a strict procession and security, as if I were entering an enemy country.”