大毘盧遮那成佛經疏 13.64

13.64

次如來藏印。先作三補吒合掌（空心也），水風二指皆屈入掌中（合指相對當），其二手火指直申而令指頭相合，二空指並而壓於風指，二手地指亦舒，勿指頭相到而稍屈之皆令曲，即是也。

Next is the Tathāgata-Garbha Seal. First make the sampuṭa añjali (hollow-palmed), with the water and wind fingers both bent into the palms (joining the fingers to face one another); straightly extend the fire fingers of both hands, joining their fingertips; place the two space fingers side by side, pressing on the wind fingers; the earth fingers of both hands are also extended, without letting their fingertips touch, and slightly bent so that both are curved; that is it.