俱舍論記 13.172

13.172

「相應善者至如雜藥水」者，釋第三句，明相應善。如雜藥水，水與藥雜，名為藥水。餘心、心所與自性善相應方成善性，故名相應善。

Regarding the passage from "Good by association" up to "like water mixed with medicine": this explains the third line, clarifying what is good by association (saṃprayukta-kuśala). Just as with water mixed with medicine, where the water is admixed with medicine and is thus called medicinal water, other minds and mental factors become good in nature only when associated with what is good by its intrinsic nature; therefore, they are called "good by association."