大毘盧遮那成佛經疏 13.203

13.203

次白傘佛頂印。散舒左手，令指頭相去各寸許，以為傘。右手作拳，而竪風指為柄，以柄柱左掌心是也（凡言作拳者，皆大指在外，他皆倣此）。真言如前耳。

Next is the Sitātapatroṣṇīṣa mudrā. Spread and extend the left hand, keeping the fingertips about one inch apart from one another, to make the parasol. Make a fist with the right hand and erect the wind finger to serve as the handle, using the handle to support the palm of the left hand (whenever 'making a fist' is said, the thumb is always on the outside; all others follow this). The mantra is as before.