《资治通鉴·唐纪·唐纪四十三》
北庭、安西自从吐蕃陷落河、陇以来，便与朝廷隔绝不通了。伊西、北庭节度使李元忠、四镇留后郭昕率领将士严守四境，抗拒吐蕃，屡次派遣使者上表，都未到达，音信断绝长达十余年。
北庭、安西自吐蕃陷河、陇，隔绝不通，伊西、北庭节度使李元忠、四镇留后郭昕帅将士闭境拒守，数遣使奉表，皆不达，声问绝者十馀年。
Since Tufan captured He and Long, Beiting and Anxi have been cut off from the imperial court. Li Yuanzhong, the military governor of Yixi and Beiting, and Guo Xin, the prefect of Sizhen, led their soldiers to guard the four borders and resisted Tufan. They sent envoys to the imperial court many times, but none of them arrived, and the news was cut off for more than ten years.