Śatapatha Brāhmaṇa ("of a hundred paths") 7.4.1.5

7.4.1.5

tathaivaitadyajamānaḥ | yatsatyaṃ sāma purastādgāyati tadasya satyam mukhaṃ karoti sa satyam bhavati satyamenamanuvartate satyo 'sya sa kāmo bhavati yatkāma etatkurute

And in like manner when the Sacrificer now, at the outset, sings the 'true hymn,' he thereby makes the truth his (Agni's) mouth; and he (himself) becomes the truth; and truth turns unto him; and true becomes that wish of his for which he performs this rite.