大毘盧遮那成佛經疏 14.4

14.4

次以左手握地水指入掌，火風相並而申之，其空指如尋常作拳也，此黑夜天印也。閻羅王后也。

Next, with the left hand, clasp the earth and water fingers into the palm, extend the fire and wind fingers side by side, and let the space finger form a fist as usual; this is the Kālarātri Devī mudrā, the queen of King Yama.