《汉书·传·匈奴传上》
孝文帝后元二年，汉朝派使者送书信给匈奴说： 皇帝敬问匈奴大单于平安无恙。
孝文后二年，使使遗匈奴书曰： 皇帝敬问匈奴大单于无恙。
In the second year of Emperor Xiaowen's Houyuan reign period, the Han court sent an envoy to the Huns with a letter, "I, your humble servant, the Son of Heaven, respectfully inquire after the health and well-being of the great chanyu of the Huns."