Yogācārabhūmi-śāstra, Volume 34 (16)

Original text:云何寻思内事劬劳。所作变异。无常之性。谓由观见。或自或他身疲劳性。身疲极性。或驰走所作。或跳踊所作。或趒踯所作。或骗骑所作。或作种种迅疾身业。复于余时。见彼远离疲劳疲极。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 internal matters of the body — the fatigue from exertion, the arising of changes, and the nature of impermanence? One observes the fatigability of one's own body or another's body, the extreme exhaustion of the body, whether it is caused by rapid walking, by jumping, by repeated leaping, by riding a horse, or by performing various swift bodily actions; and then, at a later time, one sees that this extreme fatigue has ceased and disappeared. Upon observing these phenomena, the following thought arises in the mind: Such volitional formations are truly impermanent in nature.