Yogācārabhūmi-śāstra, Volume 34 (22)

Original text:云何观察内事疾病。所作变异。无常之性。谓由观见。或自或他先无疾病。安乐强盛。后时观见。或自或他遭重病苦。触对猛利。身诸苦受。如前广说。复于余时。还见无病。安乐强盛。见是事已。便作是念。如是诸行。其性无常。

Explanation: How does one observe the impermanent nature of the changes brought about by illness in internal bodily phenomena? One observes that oneself or others previously had no illness and were at ease, healthy, and vigorous; later, one sees that oneself or others have suffered from severe illness and intense pain, encountering sharp and violent bodily feelings of suffering, as extensively described before. Then, at another time, one again sees that oneself or others are free of illness, once more at ease, healthy, and vigorous. After observing these phenomena, the thought arises in the mind: such conditioned formations are truly impermanent in nature.