Shatapatha Brahmana · Chapter 14.8.5

hṛdayabrahmaṇaḥ satyamityupāsanāvidhibrāhmaṇam
tadvai tadetadeva tadāsa/ satyameva sa yo haivametanmahadyakṣaṃ prathamajaṃ veda satyaṃ brahmeti jayatīmāṃllokānjita innvasāvasadya evametanmahadyakṣamprathamajaṃ veda satyaṃ brahmeti satyaṃ hyeva brahma - १४.८.५