大毘盧遮那成佛經疏 13.50

13.50

此中有訶聲即是因也，兼有三昧義。此係是呼召聲也，謂呼此成佛之因也。此因以本不生故離因果，即令此因淨復淨。

Here, having the sound 'ha' is the cause, and it also contains the meaning of samādhi. This 'he' is a vocative sound, meaning summoning this cause of buddhahood. Because this cause is originally unborn, it is free from cause and effect, which makes this cause purified and purified again.