大毘盧遮那成佛經疏 13.27

13.27

次金剛不壞坐。印法先以二空指二地指皆相並，餘舒散如開剖之蓮，經云令如鐸形也。次即改以地及空指相捻，中間六指並如舊舒散，其中火風指相合，水指單立即是也。此是蓮華之坐，亦名金剛座。猶坐此座故能生諸佛。諸佛皆由此坐，即名吉祥之坐也，故名金剛不壞座也。真言歸命如前。

Next is the vajra indestructible seat. As for the mudrā ritual: first place the two space fingers and the two earth fingers side by side, with the rest spread open like an opened lotus; the sūtra says to make it like the shape of a bell. Next, alter it by pinching the earth and space fingers together; the six fingers in between are spread open as before, among which the fire and wind fingers are joined together, and the water fingers stand alone—this is it. This is the lotus seat, also called the vajrāsana. Because of sitting upon this seat, one can give birth to all buddhas. All buddhas arise from this seat, so it is named the auspicious seat; hence it is called the vajra indestructible seat. The mantra's homage is as before.