俱舍論記 22.77

22.77

「又契經言至觀樂為苦」者，通第二經。經言以苦觀樂受者，此苦即是無常行苦、變壞壞苦。樂受有二：一有樂性、二有苦性。觀樂過患，佛不勸觀。觀苦利益，故佛勸觀，非無有樂。又解：無常、變壞俱是行苦。

As for "Furthermore, a sūtra says... view pleasure as suffering": this resolves the second sūtra. When the sūtra says that one should view pleasant feeling as suffering, this suffering is precisely the suffering of conditioned existence, which is impermanence, and the suffering of deterioration, which is decay. Pleasant feeling has two aspects: first, it has the nature of pleasure; second, it has the nature of suffering. With respect to viewing the faults of pleasure, the Buddha did not urge viewing [them as pleasure]; with respect to the benefit of viewing suffering, the Buddha therefore urged viewing [it as suffering]; it is not that there is no pleasure. Another interpretation: Impermanence and deterioration are both the suffering of conditioned existence.