大毘盧遮那成佛經疏 13.31

13.31

次佛頂印。先作指向內拳，即申二中指令相並，次以二風指加於中指之背，而屈二空指令相並，即是也（其指向內拳，即是指頭向內也）。此大印名佛頂，結此法印時即同仁者也。仁者謂如來也，謂作此印即同諸佛身也。此是真言，亦是具三昧解脫義也。初為因，後為果。因是如來之行，果即是佛也（合加頂上。更問）。

Next is the uṣṇīṣa mudrā. First make the fingers-inward fist, then extend the two middle fingers so they are side by side; next, place the two wind fingers on the backs of the middle fingers, and bend the two space fingers so they are side by side—this is it (the fingers-inward fist means that the fingertips point inward). This great mudrā is called the uṣṇīṣa; when forming this ritual mudrā, one is identical to the Benevolent One. The Benevolent One refers to the Tathāgata; that is, forming this mudrā is identical to the body of the buddhas. This is the mantra, and it also embodies the meaning of samādhi and liberation. The beginning is the cause, and the end is the fruit. The cause is the Tathāgata's practice, and the fruit is the Buddha (it should be placed over the crown of the head; inquire further).