大乘起信論廣釋卷第三．四．五 4.112

4.112

論。眾生亦爾至心實不動　釋曰。此法合也。推求動念已滅未生中無所住。故無有起。則知心性實不動也。

[Treatise:] Sentient beings are also like this: [... the mind is actually unmoving].
[Commentary:] This is the application to the teaching. Examining the moving thoughts, having already ceased and not yet arisen, there is no place in between where they abide; thus there is no arising at all. One then knows that the mind-nature is actually unmoving.