大般涅槃經義記 5.191

5.191

就下舉常顯無常中，句別有五：一明虛空無為故常，此名法性以為虛空，非太虛也；二明佛性無為故常；三以空即性；四以佛性即已如來，如來即無為，無為即常；五以常即法，法即是僧，僧即無為，無為即常。

Within adducing the permanent to reveal the impermanent below, there are specifically five statements: first, clarifying that space is permanent because it is unconditioned—this names the dharma-nature as space, not the physical sky; second, clarifying that the buddha-nature is permanent because it is unconditioned; third, that emptiness is identical to the nature; fourth, that the buddha-nature is identical to the Tathāgata, the Tathāgata is unconditioned, and the unconditioned is permanent; fifth, that the permanent is identical to the Dharma, the Dharma is identical to the Saṅgha, the Saṅgha is unconditioned, and the unconditioned is permanent.