Śatapatha Brāhmaṇa ("of a hundred paths") 3.8.2.10

3.8.2.10

tadyatkrūrīkurvanti | yadāsthāpayanti śāntirāpastadadbhiḥ śāntyā śamayatastadadbhiḥ saṃdhattastatte śudhyatviti tanmedhyaṃ kurutaḥ śasahobhya iti jaghanena paśuṃ ninayataḥ

Thus, wherever they wound it, wherever they hurt it--water being a means of soothing--they soothe it by that means of soothing, water, there they heal it with water: 'May that become pure in thee!' thereby they render it sacrificially pure. With 'Auspicious be the days!' they pour out (the remaining water) behind the victim.