瑜伽論記 18.249

18.249

言又此行苦遍行一切三受中者，三受行時為彼麁重所，皆是行苦。若爾，云何一偏說捨受是其行苦，為釋此疑故，云然於捨受此麁重性分明顯現，但說捨受行苦故。

Regarding "Furthermore, this suffering of conditioned existence operates universally within all three feelings": when the three feelings operate, being [pervaded] by that coarse heaviness, all are the suffering of conditioned existence. If so, why is neutral feeling alone said to be the suffering of conditioned existence? To resolve this doubt, it says: "However, because this coarse-heavy nature is clearly manifested in neutral feeling, only neutral feeling is spoken of as the suffering of conditioned existence."