Yogācārabhūmi Śāstra, Volume Thirty-Four (19)

Original text:云何寻思内事威仪。所作变异无常之性。谓由观见。或自或他行住坐卧。随一威仪。或时为损。或时为益。见是事已。便作是念。如是诸行。其性无常。余如前说。

Explanation: How does one contemplate the impermanent nature of the changes occurring in the internal matters of the body with respect to the four postures? By observing that oneself or others, while walking, standing, sitting, or lying down, in any one of these postures, may at times suffer impairment and at times gain benefit. Having observed these phenomena, such a notion arises in the mind: such volitional formations are truly impermanent in their nature.