大毘盧遮那成佛經疏 20.13

20.13

第十受食火名劫微，謂火施時受飲食之義也。受食者，謂施火食時受而食之（更問）。其尊作持唵字印者，即是梵志儀法也。淨行者凡有所言語，皆側右手作印形而舉案之，以唵字聲而作相故也。

The tenth, the fire that consumes offerings, is named Kavyavāhana, which means receiving food and drink during the fire offering. 'Consuming food' means that when food is offered to the fire, he receives and consumes it (inquire further). That this deity forms the mudrā holding the syllable OṂ is the rite of the brahmins. Whenever a pure practitioner speaks, he tilts his right hand into a mudrā, raising and pressing it, forming the sign with the sound of the syllable OṂ.