《新唐书·列传·卷一百四十一》
第二年朝廷派散骑常侍李之芳、太子左庶子崔伦前去聘问，吐蕃留下他们不让回去。
明年，使散骑常侍李之芳、太子左庶子崔伦往聘，吐蕃留不遣。
The following year, the court sent the dispersed regular attendant Li Zhifang and the crown prince’s second assistant Cui Lun to pay a call. Tibet detained them and did not let them return.