《资治通鉴·唐纪·唐纪六十五》
丰又向杜索回留学成都的子弟，送交给杜的牒文也出言不逊，从此以后不按时向唐朝入贡，还时常侵扰唐朝边境。
又索习学子弟，移牒不逊，自是入贡不时，颇扰边境。
<number>. Feng also demanded the return of the descendants of the tribesmen who were studying in Chengdu, and his correspondence to Du contained insulting words. From that point forward, he not only stopped sending tribute to the Tang dynasty on time, but also often sent troops to raid the Tang dynasty's border areas.