釋氏稽古略 3.140

3.140

開元二十六年。詔天下州郡各建一大寺。以紀年為號。額曰開元寺。六月立皇太子璵改名亨（唐書）。

In the twenty-sixth year of Kaiyuan, an edict ordered every prefecture and commandery in the realm to build a great monastery named after the reign era, with plaque inscribed "Kaiyuan Monastery." In the sixth month, the Imperial Crown Prince Yu was installed and his name was changed to Heng (Tangshu).