大毘盧遮那成佛經疏 13.130

13.130

次地惠幢印。先以左手作拳（亦是大指在外之拳），而申地水二指，即是也。

Next is the mudrā of Bhūmiprabhāketu. First make a fist with the left hand (also a fist with the thumb outside), and extend the two fingers of earth [little] and water [ring]; this is it.