大乘起信論義記 3.247

3.247

合中言心實不動者，推求動念，已滅未生中無所住，無所住故則無有起，故知心性實不動也。

In the application, the phrase 'the mind does not actually move' means that when one investigates the movement of thought, it has no abiding place between what has already ceased and what has not yet arisen; because it has no abiding place, there is no arising. Therefore one knows that the nature of the mind does not actually move.