《新唐书·列传·卷一百四十一》
还说灵盐节度使杜希全、泾原节度使李观，是外蕃信任的人，请一同主盟。
并言灵盐节度使杜希全、泾原节度使李观，外蕃所信，请主盟。
It also said 灵盐节度使 Du Xiquan and 泾原节度使 Li Guan were trusted by the foreigners, please invite them to be the chief negotiators.