仁王護國般若波羅蜜多經疏 1.42

1.42

三簡邪者，諸外道輩於自教初皆悉置於阿漚二字，云梵王訓世有七十二字，眾生轉薄，梵王嗔怒吞噉諸字，唯此二字在口兩角，阿表於無、漚表於有，置彼教首。今佛世尊教阿難置如是等言，簡異彼故。

Third, regarding excluding the heterodox: the various non-Buddhists all place the two syllables A and OṂ at the beginning of their own teachings, saying that King Brahmā instructed the world with seventy-two syllables, but as sentient beings grew increasingly degraded, King Brahmā became enraged and swallowed all the syllables, leaving only these two syllables at the two corners of his mouth: A representing non-existence and OṂ representing existence, which they place at the head of their teachings. Now, the Buddha, the World-Honoured One, instructed Ānanda to place words such as "Thus", and so forth, in order to distinguish [his teachings] from theirs.