瑜伽師地論略纂 8.186

8.186

論此中解無罪適悅相應云「又由彼念於所緣境無有染污，極安穩住，熟道適悅相應而轉」等者，此中意，以能除染故，得極安穩成辨之道，適悅自體而與相應，故名無罪。

In explaining association with faultless joy here, the treatise says: "Moreover, because that mindfulness has no defilement regarding the cognitive object, it abides with extreme peace and stability, proceeding in association with the joy of the ripened path," and so forth. The meaning here is that because it can eliminate defilement, one attains the path of extreme peace, stability, and accomplishment, joyfully being in association with its own nature; hence it is called faultless.