俱舍論記 28.78

28.78

「由初二樂輕安攝故」者，說一切有部答。由初、二定，樂、輕安攝故，所以第三說增樂受。

"Because the pleasure of the first two is subsumed under pliancy" is the answer of the Sarvāstivāda. Because in the first and second dhyānas, pleasure is subsumed under pliancy, the third is said to add pleasant sensation.