Yāska's Nirukta 13.10

13.10

atha paśusu/
tato yā vāc atyaricyata tām brāhmaṇeṣv adadhuh/
tasmād brāhmaṇā ubhayīm ācam vadanti yā ca devānām yā ca manuṣyānām/''.iti/
atha eṣā akṣarasya/
``ṛco akṣare parame vyoman yasmin devā adhi viśve niseduh/
yas tan na veda kim ṛcā kariṣyati ya it td vidus ta ime samāsate/''.
upadiśasi/
katamat tad etad akṣaram/
om ity eṣā vāc iti śākapūnih/
ṛcaś ca hy akṣare parame vyavane dhīyante nānā devateṣu ca mantreṣu/
``etadd ha vā etad akṣaram yat sarvām trayīm vidyām prati prati/''..iti ca brāhmaṇam/

Then in cattle.
Then the speech which was left over, that they placed in the Brāhmans.
Therefore Brāhmans speak both kinds of speech: that of the gods and that of men."
Now this [verse] is of the Imperishable [Syllable]:
"The Ṛcs are in the supreme imperishable firmament, in which all the gods have taken their seat.
He who knows that not, what will he do with the Ṛc? Those who know it sit here assembled."
You teach.
Which is that imperishable [syllable]?
"It is the syllable *oṃ*", says Śākapūṇi.
For the Ṛcs are established in the supreme imperishable firmament and in the mantras addressed to various deities.
And there is the Brāhmaṇa passage: "This indeed is that imperishable [syllable] which corresponds to each and every part of the threefold knowledge."