摩訶止觀 8.199

8.199

道品者：魔界具一切色。色即是空，色即不淨；色即是假，此名為淨；色即是中，非淨非不淨。餘四陰亦如是。是名「一念處一切念處」，乃至三解脫門。

Regarding the factors of the path: the māra-realm contains all form. Form is emptiness; form is impure. Form is the provisional; this is called pure. Form is the middle; it is neither pure nor impure. The remaining four aggregates are also like this. This is called 'one smṛtyupasthāna is all smṛtyupasthānas,' up to the three gates of liberation.