大乘義章 11.63

11.63

次就有常無常分別。小無量心一向無常，大無量心一向是常。大中義分，用隨世變名為無常，與經中說功德莊嚴有常非常其義相似；德體不變名之為常，故經中說慈即佛性常樂我淨，悲喜捨心類亦同爾。

Next, they are distinguished with respect to permanent and impermanent. The small immeasurable minds are unequivocally impermanent; the great immeasurable minds are unequivocally permanent. Within the great, the meaning is divided: their function adapting to worldly transformations is called impermanent, which is similar in meaning to the sūtras stating that adornments of virtue are both permanent and not permanent; their essence of virtue being unchanging is called permanent. Thus the sūtra states that loving-kindness is the Buddha-nature, permanent, blissful, self, and pure; the minds of compassion, sympathetic joy, and equanimity are also the same in kind.