In the Comprehensive Mirror for Aid in Government it is recorded that in the sixth year of the Dahe period of the Tang dynasty, while the emperor was at his Yan Ying Palace he asked his prime minister, Niu Sengru, when there would be peace in the world and where he could look for a sign.
《資治通鑑》上記載，唐朝大和六年，皇帝在延英殿，問宰相牛僧孺，天下何時太平？有何跡象可尋？