Śatapatha Brāhmaṇa ("of a hundred paths") 1.1.1.6

1.1.1.6

atha saṃsthite visṛjate | idamahaṃ ya evāsmi so 'smītyamānuṣa iva vā etadbhavati yadvratamupaiti na hi tadavakalpate yadbrūyādidamahaṃ satyādanṛtamupaimīti tadu khalu punarmānuṣo bhavati tasmādidamahaṃ ya evāsmi so 'smītyevaṃ vrataṃ visṛjeta

After the completion (of the sacrifice) he divests himself (of the vow), with the text (Vâg. S. II, 28 b): 'Now I am he who I really am.' For, in entering upon the vow, he becomes, as it were, nonhuman; and as it would not be becoming for him to say, 'I enter from truth into untruth;' and as, in fact, he now again becomes man, let him therefore divest himself (of the vow), with the text: 'Now I am he who I really am.'