摩訶止觀 6.74

6.74

非有非無門者，絕四離百，言語道斷，不可說示。《涅槃》云：「非常、非斷名為中道。」即是其門也。

The gate of neither existence nor non-existence cuts off the four [propositions] and departs from the hundred [negations]; the path of words is cut off, and it cannot be spoken of or shown. The Nirvāṇa [Sūtra] says: 'Neither eternal nor annihilated is called the middle way.' This is that gate.