俱舍論疏 11.130

11.130

論「樂生三者至故名樂生」，此釋樂生三天別。下三靜慮各有三天，故成九處。初定離生喜樂，二定定生喜樂，三定離下喜樂乃至長時受樂，第四禪已上無樂受故不名樂生。

The Commentary states, "From 'As for the three births of pleasure' up to 'hence named birth of pleasure,'" this explains the distinctions among the three heavens of birth of pleasure. The lower three dhyānas each have three heavens, thus forming nine places. The first dhyāna has joy and pleasure born of seclusion; the second dhyāna has joy and pleasure born of concentration; the third dhyāna has pleasure detached from the lower joy, up to experiencing pleasure for a long time. Since there is no sensation of pleasure from the fourth dhyāna upward, they are not called birth of pleasure.