俱舍論疏 7.153

7.153

論「無色染心至故名得二」，明無色染心也。彼無上界，故無界退還。起惑退時，起上不得下，退下必得上，由此無色染心唯得二心。

The treatise says: "A defiled mind of the formless realm... therefore it is said that two are acquired." This explains the defiled mind of the formless realm. There is no higher realm above it, so there is no return from a realm. When falling away through the arousal of afflictions, if one arouses an upper [defilement] one does not acquire the lower, but falling down to a lower [stage] one inevitably acquires the upper; therefore, in a formless defiled mind, only two minds are acquired.