At the end of the Yuanhe period (806-20 CE), there was a commander of Huaixi, who was sent to Bianzhou, and stopped at a courier station on the way. Late at night, when he was reaching deep sleep, he suddenly awoke to find something pressing heavily onto him. The general, well-trained and strong, leapt up in alarm and began to wrestle with it, after which the unidentified thing withdrew, and the general succeeded in wresting a leather bag from its hand. The ghost begged and implored with great bitterness from the darkness, so the commander addressed it: “If you tell me what it is, I will give it back.” The spirit said, after a long time, “This is a bag of surplus qi.” The commander then picked up a brick and struck out with it, at which the voice was silenced. The bag held several sheng (these are about a litre each), its contents were deep red in colour, and looked like lotus-root fibres; when carried in daylight it cast no shadow.
Taken from Youyang zazu.
Li Fang 李昉, et al., Taiping guangji 太平廣記 (Extensive Gleanings from the Era of Great Harmony), 10 vols (Beijing: Zhonghua shuju, 1961), vii, 345.2733: