俱舍論疏 13.166

13.166

論「欲色二界至身語律儀」，釋無色界無無表業。一以無色無大種故，二以無色無身、語轉，故無色界無無表也。

The commentary states: "The desire and form realms ... bodily and vocal restraints." This explains that the formless realm lacks avijñapti-karma. First, because the formless realm lacks great elements; second, because the formless realm lacks the operation of body and speech, the formless realm lacks avijñapti.