《后汉书·列传·班彪列传下》
班固从永平中年开始受诏，绞尽脑汁，深思二十多年，至建初年间才写成。
唐哉皇哉，皇哉唐哉。
Ban Gu began to receive the imperial decree in the middle of the Yongping era, racked his brains, and thought deeply for more than twenty years. He finally finished it in the early years of the Jianchu era.