大毘盧遮那經供養次第法疏 2.9

2.9

The gate of the mantra-hṛdaya of Vairocana: Vairocana's teaching of the mantra-hṛdaya is because, within each of the aforementioned mantras, there are mūla-mantras, hṛdaya-mantras, and upahṛdaya-mantras, immeasurable, boundless, and innumerable like this; now he speaks of them collectively as the mantra-hṛdaya. This letter A is the meaning of the original non-arising (ādy-anutpāda) of all dharmas. If separated from the sound A, there are no other letters; it is the mother of all letters, the place from which all mantras arise. That is, all dharma-gates, bodhisattvas, and so forth manifest these things through the power of adhiṣṭhāna from the mind of Vairocana's own realization, in order to benefit sentient beings. Yet in reality, their essence does not arise; it is identical to the dharma-essence of the letter A. This letter is the most supreme among mantras; therefore, a mantra practitioner should always receive and hold it in this way. For this reason, all mantras abide in the letter A; because they abide in this, reciting it brings forth the virtues of all other letters.