大般涅槃經集解 35.12

35.12

智秀曰：說常也寂滅無苦。以無苦故，名之為樂。無更別樂，故云寂滅。

Zhixiu says: Speaking of permanence, tranquil extinction is devoid of suffering. Because it is devoid of suffering, it is called bliss. There is no other distinct bliss, which is why it is called tranquil extinction.