妙法蓮華經文句 2.109

2.109

觀心解者，自行十善，勸他隨喜，此三十善皆空、皆假、皆中，即是三十三觀門也。

As for the explanation by contemplation of mind: practicing the ten good deeds oneself and encouraging others to rejoice in accord with them—these thirty good deeds are all empty, all provisional, and all middle, which is precisely the thirty-three approaches of contemplation.