俱舍論疏 28.97

28.97

論「第二靜慮至四等持別」，釋第二靜慮支也。言「信名內等淨」者，《正理》釋云「信是淨相故立淨名，如水清珠令心淨故。內心平等為緣故生，由此信根名內等淨。或第二定所有功德，平等為緣引生此淨，由此建立內等淨名（已上論文）。」

The Śāstra says: "In the second dhyāna... [the distinct four are concentration]," which explains the limbs of the second dhyāna. As for "faith is called internal purification (adhyātmasaṃprasāda)," the Nyāyānusāra explains: "Faith has the characteristic of purity, hence it is established under the name 'purification,' just as the water-clearing jewel purifies water. It arises with the internal mind's serenity (samatā) as its condition; therefore this faculty of faith is called 'internal purification.' Or else, all the merits of the second concentration equanimously act as conditions to induce this purification; hence the name 'internal purification' is established (the above is from the treatise)."