妙法蓮華經玄義 8.123

8.123

諸法既如幻化，幻化本自無實，無實故空，乃至涅槃亦如幻化，如鏡中像，假有形色，求不可得，是為空門。

Since all dharmas are like an illusion, and an illusion is fundamentally devoid of actuality, because it is devoid of actuality it is empty; even nirvāṇa is also like an illusion, like an image in a mirror provisionally having form and color, yet cannot be found when sought; this is the gate of emptiness.