Śatapatha Brāhmaṇa ("of a hundred paths") 1.1.1.1

1.1.1.1

vratamupaiṣyan | antareṇāhavanīyaṃ ca gārhapatyaṃ ca prāṅ tiṣṭhannapa upaspṛśati tadyadapa upaspṛśatyamedhyo vai puruṣo yadanṛtaṃ vadati tena pūtirantarato medhyā vā āpo medhyo bhūtvā vratamupāyānīti pavitraṃ vā āpaḥ pavitrapūto vratamupāyānīti tasmādvā apa upaspṛśati

He who is about to enter on the vow, touches water whilst standing between the Âhavanîya and Gârhapatya fires, with his face turned towards east. The reason why he touches water is, that man is (sacrificially) impure on account of his speaking untruth; and because by that act an internal purification (is effected),--for water is indeed (sacrificially) pure. 'After becoming sacrificially pure, I will enter on the vow,' thus (he thinks); for water is indeed purifying. 'Having become purified through the purifying one, I will enter on the vow,' thus (he thinks, and) this is the reason why he touches water.