摩訶止觀 6.78

6.78

空門者，觀幻化見思及一切法，不在因，不屬緣。我及涅槃，是二皆空，唯有空病，空病亦空。此即三諦皆空也。

As for the gate of emptiness, one contemplates the illusory [delusions of] view and thought and all dharmas as neither residing in causes nor belonging to conditions. Self and nirvāṇa—both of these are empty; there is only the disease of emptiness, and the disease of emptiness is also empty. This means the three truths are all empty.