It is from The Dream on the Pillow by Shen Jiji of the Tang Dynasty. The story says：
故事出自唐代沈既济的传奇小说《枕中记》，大意是说：
A discontented poor scholar named Lu Sheng yearned all day long for an official position and a distinguished career.
卢生少年家贫，一直希望有朝一日能够建功树名，出将入相。
He met at an inn in Handan a Taoist priest who gave him a porcelain pillow.
有一次，卢生偶然在邯郸的一家客店中遇见道士吕翁，吕翁给了他一只青瓷枕。
At that time, the inn host just started cooking a pan of millet.
当时，客店的主人正在蒸黄粱米饭，
Lu fell asleep while waiting for his lunch and dreamed that he married the daughter of a rich man,
卢生等着吃午饭时睡着了。在梦中，卢生娶崔氏女为妻，
succeeded in the highest imperial examinations ,
考中了进士，
became a high-ranking official,
当上了宰相，
had sons, and enjoyed a life of wealth and comfort.
子孙满堂，享尽荣华富贵。
When waking up, he found that the pot of millet was still cooking on the fire.
一觉醒来之后，却发现客店主人的黄粱米饭尚未蒸熟.
Now the story is meant to satirize the nullity of fantasies and unfeasible hope. Another phrase is “golden dreams on the porcelain pillow".
后人便人便用“黄粱美梦”一词来比喻虚幻的好事或不切实际的希望。
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