Lankavatara Sutra, Chapter 5, Section LXXXI

As usual, I'm using the translation here.  And I'm trying, best as I can, to summarize it.  It's not to be taken as any kind of "teaching."  No teaching on this blog, you know.

This is about the issue of whether the state of being a Tathagata is permanent or not. You can guess the Buddha's answer by now: the Tathagata is neither permanent nor impermanent.

IF "permanent" → related to creating agency [ where creating agencies are not created and permanent themselves] → preparation for Tathagatahood (i.e., practice and training) are useless. Further, since there is not no birth applied to the Tathagata, such a being is not permanent.

Also, the categories "permanent" and "impermanent" imply dualism, which is not a category to be applied to the Tathagata.

But aside from that, the Tathagata is not impermanent because:

[T]he knowledge arising from the attainment of enlightenment [ = an intuitive understanding] is of a permanent nature, the Tathagata is permanent. Mahāmati, this knowledge, as it is attained intuitively by the Tathagatas, Arhats, Fully-Enlightened Ones, is, indeed, permanent. Whether the Tathagatas are born or not, this Dharmatā, which is the regulative and sustaining principle to be discoverable in the enlightenment of all the Śrāvakas, Pratyekabuddhas, and philosophers, abides, and this sustaining principle of existence is not like the emptiness of space, which, however, is not understood by the ignorant and simple-minded. Mahāmati, this knowledge of enlightenment belonging to the Tathagatas comes forth from transcendental knowledge (prajñājñāna); Mahāmati, the Tathagatas, Arhats, Fully-Enlightened Ones do not come forth from the habit-energy of ignorance which is concerned with the Citta, Manas, and Manovijñāna, and the Skandhas, Dhātus, and Āyatanas. The triple world originates from the discriminating of unrealities, but the Tathagatas do not originate from the discriminating of unrealities. Where duality obtains, Mahāmati, there is permanency and impermanency because of its not being one.

And so it is with Chapter 5.