大毘盧遮那成佛經疏 13.30

13.30

次作金剛印。二手皆屈水指向掌內，以二風指捻火指背而不相合，此即金剛印也（二空指並而竪也）。由此金剛印，能壞無智之城，一切不能壞也。其真言歸命如前　呪體但有吽（引）字（義如上准說之）。

Next, form the vajra mudrā. With both hands, bend the water fingers into the palms, touch the backs of the fire fingers with the two wind fingers without joining them—this is the vajra mudrā (the two space fingers are side by side and erect). By means of this vajra mudrā, one can destroy the citadel of ignorance, and nothing can destroy it. Its mantra's homage is as before; the body of the dhāraṇī consists only of the syllable *hūṃ* (drawn out) (its meaning is explained following the above).