Shatapatha Brahmana 1.7.1.18

1.7.1.18

athottamāṃ dohayitvā | yena dohayati pātreṇa tasminnudastokamānīya palyaṅya pratyānayati yadatra payaso 'hāyi tadihāpyasaditi rasasyo caiva sarvatvāyedaṃ hi yadā varṣatyathauṣadhayo jāyanta oṣadhīrjagdhvāpaḥ pītvā tata eṣa rasaḥ sambhavati tasmādu rasasyo caiva sarvatvāya tadudvāsyātanakti tīvrīkarotyevainadetattasmādudvāsyātanakti

After having the last (cow) milked, and having poured a drop of water into the pail which he has made the milker use, and stirred it, he pours it to (the milk), thinking 'what milk was left there, let that also be here!'--(he does so) for the completeness of the sap; for when it rains here, then plants spring up, and on the plants being eaten and the water drunk, thence is this juice produced: and therefore (the water is poured to the milk) for the completeness of the sap. Having then taken it off (the fire), he coagulates it: he thereby makes it sharp (pungent); for this reason he coagulates it, after taking it off (the fire).