大毘盧遮那成佛經疏 10.61

10.61

次一切持金剛真言。時十佛剎塵數金剛同入金剛無勝三昧，猶如金剛手無異，亦於自心出不思議光，光中現此真言。當知餘眷屬准有也。吽吽吽。以初字為體，亦是大空行三昧也。此三昧即是大金剛無勝之行人也。入此三昧故名大空，行三昧也。三字是眾多義，一切眾生多金剛同說，故合三字也。

Next is the mantra of all vajradharas. At that time, vajradharas as numerous as the dust motes of ten buddha-kṣetras entered together into the vajra invincible samādhi, no different from Vajrapāṇi, and likewise emanated inconceivable light from their own minds, in which this mantra appeared. One should know that the rest of the retinue is also likewise. *Hūṃ hūṃ hūṃ*. It takes the first syllable as its essence; it is also the great emptiness practice samādhi. This samādhi is none other than the practitioner of the great vajra invincibility. Entering this samādhi, it is therefore named the great emptiness practice samādhi. The three syllables indicate multiplicity; since all sentient beings' many vajras spoke it together, the three syllables are therefore combined.