Li Bai's only post on the upper terrace, why is it sure that it was written by Li Bai?

This "Shangjietie" is recognized as the only original calligraphy handed down by Li Bai. After generations of mounting, this work has become a long scroll in front of people.

This volume was returned to the imperial palace during the reign of Song Xuanhe. Then it flowed into the hands of Jia Sidao at the end of Song Dynasty; In the Yuan Dynasty, it passed through Zhang Qian's office, and in the Ming Dynasty it was hidden in Xiangtian Pavilion. In the Qing dynasty, in order to collect everyone's income from Angie, it was later incorporated into the internal affairs of the Qing court. It was collected by collector Zhang Boju in the Republic of China, and now it is collected by Beijing Palace Museum.