摩訶止觀 8.193

8.193

欲滿此願、顯此理，應降魔作道場。八十億眾不能動心，名「止」；達魔界即佛界，名「觀」，但以四悉，止觀安心。

To fulfill this vow and manifest this principle, one should subjugate māras to make the bodhimaṇḍa. That eighty koṭīs of multitudes cannot move one's mind is called 'calming'; penetrating that the māra-realm is itself the Buddha-realm is called 'contemplation'; one merely pacifies the mind through calming and contemplation by means of the four siddhāntas.