金剛經纂要刊定記 2.173

2.173

二、觀受是苦者。受即是心所遍行五中一也，仍有三種。謂：苦、樂、捨。苦謂苦苦，樂謂壞苦，捨謂行苦。

Second, "contemplating sensation as suffering": sensation is one among the five omnipresent mental factors, and has three kinds: namely, suffering, pleasure, and equanimity. Suffering refers to the suffering of suffering (duḥkha-duḥkhatā); pleasure refers to the suffering of change (vipariṇāma-duḥkhatā); equanimity refers to the suffering of conditioned existence (saṃskāra-duḥkhatā).