Yogācārabhūmi Śāstra, Volume Thirty-Four (24)

Original text:云何观察内事一切不现尽灭。所作变坏。无常之性。谓由观见。彼于余时此骨锁位。亦复不现。皆悉败坏。离散磨灭。遍一切种。眼不复见。见是事已。便作是念。如是诸行。其性无常。何以故。如是色相数数改转。前后变异现可得故。

Explanation: How does one observe that all dharmas of the internal bodily matters no longer appear, have all completely ceased, and that what has manifested is subject to decay and the nature of impermanence? One observes that at a later time even the pile of bones remaining from the body is gone, all having decayed, dispersed, and worn away; every kind of material object can no longer be seen by the eyes. After observing these phenomena, the following thought arises in the mind: such volitional formations are truly impermanent in nature. Why is this said? Because the physical features of the corpse after death undergo constant transformation, and the phenomena of change from before to after can all be directly observed in the present.