Shatapatha Brahmana 6.8.1.11

6.8.1.11

yadvevaitadyajurjapati | yasminvai kasmiṃścāhite 'kṣa utsarjati tasyaiva sā vāgbhavati tadyadagnāvāhite 'kṣa utsarjatyagnereva sā vāgbhavatyagnimeva taddevā upāstuvannupāmahayaṃstathaivainamayametadupastautyupamahayatyakrandadagni stanayanniva dyauriti tasyokto bandhuḥ

And, again, why he mutters that prayer;--with whomsoever, mounted (on a chariot), the axle creaks, this is his own voice: hence when the axle creaks while Agni is mounted, this is the voice of Agni himself. It was Agni indeed whom the gods thereby praised and magnified; and in like manner does this (Sacrificer) thereby praise and magnify him: 'Agni roared like the thundering sky,'--the meaning of this has been explained.