The Vedanta Sutras, commentary by Sankaracharya (SBE34), tr. by George Thibaut [1890] at sacred-texts.com

29. And from this very reason there follows the eternity of the Veda.

As the eternity of the Veda is founded on the absence of the remembrance of an agent only, a doubt with regard to it had been raised owing to the doctrine that the gods and other individuals have sprung from it. That doubt has been refuted in the preceding Sûtra.--The present Sûtra now confirms the, already established, eternity of the Veda. The eternity of the word of the Veda has to be assumed for this very reason, that the world with its definite (eternal) species, such as gods and so on, originates from it.--A mantra also ('By means of the sacrifice they followed the trace of speech; they found it dwelling in the rishis,' Rig-veda Samh. X, 71, 3) shows that the speech found (by the rishis) was permanent.--On this point Vedavyâsa also speaks as follows: 'Formerly the great rishis, being allowed to do so by Svayambhû, obtained, through their penance, the Vedas together with the itihâsas, which had been hidden at the end of the yuga.'