"There was a rosy fairyland that once lived in Beituo, and a rainbow shadow still lies in the south." Whose poem is it?

Forty miles south of Jiangyin City, south of Nanyangqi Village, there is a couplet engraved on the rafters of the ancient stone bridge: "There was a fairy Xia who lived in Beituo, and the rainbow shadow still lies in Nanyang." It is said that there once was a man. Xia Xian lives in the north of Shiqiao. Although he has passed away, people still seem to be able to see him, like a rainbow, always floating in the sky of Nanyangqi. The Xiaxian here is talking about Xu Xiake.