大毘盧遮那成佛經疏 10.35

10.35

次地藏菩薩真言。時此菩薩入不可壞金剛行三昧。金剛者即是菩提之心，此菩提心即是不可壞，依此進行是金剛行也。

Next is the mantra of Kṣitigarbha Bodhisattva. At that time, this bodhisattva enters the Indestructible Vajra-Practice Samādhi. 'Vajra' is the bodhicitta; this bodhicitta is indestructible, and progressing by relying on this is the vajra-practice.