俱舍論疏 5.77

5.77

論「應如是知至往色界受生」，結證所以也。若不退起色界煩惱，即不得生意成天中，已離無所有惑得此定故。既經說生彼，故知有退。

Regarding the treatise's statement, "It should be understood in this way ... goes to take birth in the form realm": This concludes the proof of the reason. If one did not retrogress and give rise to the defilements of the form realm, one could not be born among the mind-made deities (manomayadeva), because this attainment is obtained by having already separated from the defilements of [the sphere of] nothingness. Since the sūtra speaks of taking birth there, we know that there is retrogression.