大毘盧遮那成佛經疏 13.32

13.32

次如來豪相印。以智手為出大指之拳，置於眉間，即是也。作此印即同如來豪相具足也。其真言歸命如前。

Next is the Tathāgata's ūrṇā mudrā. Form a fist with the thumb protruding using the wisdom hand and place it between the eyebrows—this is it. Forming this mudrā is identical to being endowed with the Tathāgata's ūrṇā. Its mantra's homage is as before.