瑜伽師地論略纂 12.172

12.172

論云「如諸聲聞善觀察所知無相三摩地加行住，菩薩第九住亦爾」者，三空無色，觀心微細，名無相三摩地。即第四禪心趣向三空故，是三空無相之加行道故，似菩薩第九住。第十成滿無相住，准在三空，如理可解。

The treatise states: "Just as all śrāvakas abide in the preparatory path of the signless samādhi that well observes the knowable, so too is the bodhisattva's ninth abiding." In the three concentrations of emptiness and the formless [realms], the observing mind is subtle, and this is named the signless samādhi. Namely, because the mind of the fourth dhyāna tends towards the three concentrations of emptiness, it is the preparatory path to the signless of the three concentrations of emptiness; hence, it is like the bodhisattva's ninth abiding. By analogy, the tenth abiding of perfected signlessness is in the three concentrations of emptiness, as can be understood in accordance with the principle.