大毘盧遮那成佛經疏 10.212

10.212

次毘盧遮那說真言心者，以如上諸真言等，隨一一中則有根本真言、心真言、隨心真言，如是等無量無邊不可知數，今總說諸真言之心，即此阿字是也。此是諸法本不生義，若離阿聲則無餘字，即是諸字之母，即一切真言生處也。謂一切法門及菩薩等，皆從毘盧遮那自體自證之心，為欲饒益眾生，以加持力而現是事能，實即體不生，同於阿字之法體也。此字於真言中最為上妙，是故真言行者常當如是受持也。是故一切真言住於阿字，猶住此故，誦之即生也。右普通真言藏品竟。此中真言通一切處用，下諸法中各是別行也。

Next, regarding Vairocana's expounding of the heart of mantras: because in each and every one of the various mantras above there is a fundamental mantra, a heart mantra, and a secondary heart mantra, and countless and limitless numbers of such kinds that cannot be known, now he speaks generally of the heart of all mantras, which is precisely this syllable A. This is the meaning of all dharmas being originally unborn. If separated from the sound A, there are no other syllables; it is the mother of all syllables, the birthplace of all mantras. That is to say, all Dharma gateways, bodhisattvas, and so forth, through the power of adhiṣṭhāna, manifest these capacities for the sake of benefiting sentient beings from the mind of Vairocana's own essence and self-realization; in reality, their essence is unborn, identical with the dharmatā of the syllable A. This syllable is the most supreme and subtle among mantras; therefore, mantra practitioners must constantly uphold it in this way. For this reason, all mantras abide in the syllable A; precisely because they abide in this, reciting it brings them to birth. The Chapter on the Treasury of Common Mantras above is concluded. The mantras herein are used universally in all places; in the methods below, each is practiced separately.