Visuddhimagga XIV-88

Lokuttaraṃ catumaggasampayogato catubbidhanti evaṃ tāva kusalaviññāṇameva ekavīsatividhaṃ hoti.

（四）（出世间善心）出世间善心，因为与（（18）须陀洹，（19）斯陀含、（20）阿那含、（21）阿罗汉）四道相应而为四种。如是先说善的识只有二十一种。[PTS 454]

Ñ(XIV,88): I. D. The supramundane is fourfold (18)-(21) by association with the four paths. So firstly, profitable consciousness itself is of twenty-one kinds. [454]