Unlike a mantra addressed to a specific deity, om ah hum does not appear fixed to one sutra or tantra of origin; it recurs across the Buddhist tantric literature from roughly the eighth century CE onward as a functional unit rather than as a quotation from any single source, used to consecrate offerings, to bless food before it is eaten, and to mark the crown, throat and heart of a visualized deity or of the practitioner's own body in generation stage practice. Standard Tibetan Buddhist symbolic explanation, set out in ritual handbooks and ordinary teaching alike, assigns the three syllables to enlightened body, speech and mind respectively, om white at the crown, ah red at the throat, hum blue at the heart, a color and location scheme that is remarkably stable across different lineages even though the deities being visualized vary enormously between them.
Hum in particular carries a function beyond this triad worth separating out: it recurs at the end of a very large number of otherwise unrelated wrathful and protective mantras across the Tibetan canon, functioning almost as a seal or binding syllable closing an invocation, a role distinct from its place here as mind's syllable within the body speech mind triad. Scholars of Buddhist tantra have discussed whether this sealing function and the mind syllable function are the same use of hum wearing two descriptions or whether the tradition has, over centuries of transmission across many independent textual streams, folded two originally distinct roles for one sound into a single symbolic package, and the scattered textual record does not obviously settle which is closer to the truth.