《明史·志·卷五十六》
正统三年，宁夏总兵官史昭因边防军队缺马，而延庆、平凉的官吏军民多养马，便上奏请求交纳马匹以换盐。
正统三年，宁夏总兵官史昭以边军缺马，而延庆、平凉官吏军民多养马，乃奏请纳马中盐。
In the third year of Zhengtong, Ningxia General Officer Shi Zhao found that the border troops lacked horses, so officials, the military, and civilians in Yanqing and Pingliang raised many horses. He memorialized and requested to exchange horses for salt.