Śatapatha Brāhmaṇa ("of a hundred paths") 4.4.3.13

4.4.3.13

atha pūrṇapātrāntsamavamṛśanti | yāneke 'psuṣomā ityā cakṣate yathā vai yukto vahedevamete ya ārtvijyaṃ kurvantyuta vai yuktaḥ kṣaṇute vā vi vā liśate śāntirāpo bheṣajaṃ tadyadevātra kṣaṇvate vā vi vā liśante śāntirāpastadadbhiḥ śāntyā śamayante tadadbhiḥ saṃdadhate tasmātpūrṇapātrāntsamavamṛśanti

Thereupon they touch the vessels filled (with water) which some call Apsushomâh (Soma-draughts in water). For even as a yoked (animal) draws, so do they who perform the priestly duties. But the yoked (animal) galls or scratches itself; and water is a means of soothing, a medicine: thus wherever in this (sacrifice) they gall or scratch themselves--water being a means of soothing--they soothe by that means of soothing, water; they heal it by water. This is why they touch the vessels filled (with water).