Śatapatha Brāhmaṇa ("of a hundred paths") 3.1.2.10

3.1.2.10

atha snāti | amedhyo vai puruṣo yadanṛtaṃ vadati tena pūtirantarato medhyā vā āpo medhyo bhūtvā dīkṣā iti pavitraṃ vā āpaḥ pavitrapūto dīkṣā iti tasmādvai snāti

He bathes. For impure, indeed, is man: he is foul within, in that he speaks untruth;--and water is pure: he thinks, 'May I be consecrated, after becoming pure;'--and water is cleansing: he thinks, 'May I become consecrated after being cleansed!' This is the reason why he bathes.