Jaiminīya Upaniṣad Brāhmaṇa 2.2.5

2.2.5

atha yas so 'pāna āsīt sa bṛhaspatir abhavat
yad asyai vāco bṛhatyai patis tasmād bṛhaspatiḥ /

Then that downward breath which was there became Bṛhaspati;
because it is the lord (pati) of this great (bṛhatī) speech, therefore it is Bṛhaspati.