Śatapatha Brāhmaṇa ("of a hundred paths") 2.2.2.20

2.2.2.20

tadu hāpyaruṇamaupaveśiṃ jñātaya ūcuḥ | sthaviro vā asyagnī ādhatsveti sa hovāca te maitadbrūtha vācaṃyama evaidhi na vā āhitāgninānṛtaṃ vaditavyaṃ na vadañjātu nānṛtaṃ vadettāvatsatyamevopacāra iti

Now the kinsmen spake unto Aruna Aupavesi, 'Thou art advanced in years: establish thou the two fires!' He replied, 'Speak ye not thus! be thou a restrainer of speech; for he who has established the fires must not speak an untruth: let him rather not speak at all, but let him not speak an untruth. Worship, above all, is truthfulness.'