摩訶止觀 1.60

1.60

若其心念念五塵六欲，外樂蓋微，三禪樂如石泉，其樂內重，此發梵心、行色無色道。

If in thought after thought one regards the five sense objects and six desires as trivial external pleasures, and the pleasure of the third dhyāna like a spring in rock, its joy profound within, this is generating the Brahmā mind and traversing the paths of the form and formless realms.