俱舍論疏 15.141

15.141

論「由下三定至說名不動」，答也。初禪有尋、伺等，第二禪有喜等，第三禪有樂，出、入息等，故名為動。今言不動，據感得不動異熟也。

The Śāstra says: "Because the lower three absorptions [...] is called immovable." This is the answer. The first dhyāna has vitarka and vicāra, the second dhyāna has joy, and the third dhyāna has pleasure, inhalation, and exhalation; hence they are called movable. Here, saying 'immovable' is based on their bringing about immovable vipāka.