Śatapatha Brāhmaṇa ("of a hundred paths") 2.2.2.19

2.2.2.19

tasya vā etasyāgnyādheyasya | satyamevopacāraḥ sa yaḥ satyaṃ vadati yathāgniṃ samiddhaṃ taṃ ghṛtenābhiṣiñcedevaṃ hainaṃ sa uddīpayati tasya bhūyo bhūya eva tejo bhavati śvaḥ śvaḥ śreyānbhavatyatha yo 'nṛtaṃ vadati yathāgniṃ samiddhaṃ tamudakenābhiṣiñcedevaṃ hainaṃ sa jāsayati tasya kanīyaḥ kanīya eva tejo bhavati śvaḥ śvaḥ pāpīyānbhavati tasmādu satyameva vadet

Now, attendance on (or, the worship of) that consecrated fire (agnyâdheya) means (speaking) the truth. Whosoever speaks the truth, acts as if he sprinkled that lighted fire with ghee; for even so does he enkindle it: and ever the more increases his own vital energy, and day by day does he become better. And whosoever speaks the untruth, acts as if he sprinkled that lighted fire with water; for even so does he enfeeble it: and ever the less becomes his own vital energy, and day by day does he become more wicked. Let him, therefore, speak nothing but the truth.