俱舍論頌疏論本 28.37

28.37

內淨即信根者，謂若證得第二靜慮，則於定地亦可離中有深信生，名內等淨。信是淨相，故立淨名。離外均流故，名內等。淨而內等故，立內等淨名。

Regarding "Internal purity is just the faculty of faith": this means that if one attains the second dhyāna, deep faith arises towards that concentration level being also capable of being departed from; this is called 'internal serenity.' Faith is the characteristic of purity, hence it is established with the name 'purity.' Being separated from external flows and becoming even, it is called 'internal evenness.' Being pure and internally even, it is established with the name 'internal serenity.'