金剛三昧經論 2.128

2.128

經曰　大力菩薩言：「云何三空？」佛言：「三空者，空相亦空、空空亦空、所空亦空。如是等空，不住三相、不無真實，文言道斷、不可思議。」

The sūtra states: Mahābalavān Bodhisattva said: "What is the threefold emptiness?" The Buddha said: "The threefold emptiness is: the mark of emptiness is also empty; the emptiness of emptiness is also empty; that which is emptied is also empty. Emptinesses such as these do not abide in the three marks and are not devoid of the real. The path of words and language is cut off, inconceivable."