Who knows that "the bright moon is not allowed to cross the river, and Hong Tao moves." Where did this sentence come from?

Gong Zizhen herself is a dutiful son. When he was living in Beijing, he wrote such a poem "A Hundred Rivers Remains to the Title" after receiving a letter from his mother. The poem wrote: "The infamous Huang Dan secretly mourned himself, singing loudly that the bright moon did not sink into the sky, and Jiang Tao was moved." Because my mother is old and her calligraphy is scrawled, Mr. Ding An cries out from the heart, hoping to arouse the boundless strength of the rivers and seas and keep her youth. My mother and son are affectionate and sincere, why do they say unfilial?