菩薩瓔珞本業經 1.38

1.38

「佛子！八、信喜大法，所謂三空：一切因空故無作，一切果空故無相，因空果空復空故空空。如是法，如虛空故。

"Buddha-son! Eighth is to believe in and delight in the Great Dharma, that is to say, the three emptinesses: because all causes are empty, there is the wishless; because all results are empty, there is the signless; and because the emptiness of cause and the emptiness of result are themselves empty, there is the emptiness of emptiness. Such phenomena are like empty space.