大乘起信論義記 3.436

3.436

亦常不得隨心外念境界，後以心除心。心若馳散，即當攝來住於正念。是正念者，當知唯心，無外境界。即復此心亦無自相，念念不可得。

One must also never follow the mind’s outward thinking of perceptual objects, nor subsequently eliminate mind with mind. If the mind becomes scattered and distracted, one should immediately gather it in to dwell in right mindfulness. As for this right mindfulness, one should know that it is mind-only, with no external perceptual objects. Furthermore, this mind itself also has no self-characteristic, and is unobtainable from thought to thought.