Śatapatha Brāhmaṇa ("of a hundred paths") 4.4.5.10

4.4.5.10

sa yaḥ syandamānānāṃ sthāvaro hradaḥ syāt | tamapo 'bhyaveyādetā vā apāṃ varuṇagṛhītā yāḥ syandamānānāṃ na syandante varuṇyo vā avabhṛtho nirvaruṇatāyai yadyu tā na vindedapi yā eva kāścāpo 'bhyaveyāt

Where there is a standing pool of flowing water, there let him (the sacrificer) descend into the water--for whatsoever parts of flowing water flow not, these are holden by Varuna; and the expiatory bath belongs to Varuna--to free himself from Varuna. But if he does not find such, he may descend into any water.