Śatapatha Brāhmaṇa ("of a hundred paths") 2.3.1.16

2.3.1.16

tadavajyotayati | śṛtaṃ vedānītyathāpaḥ pratyānayati śāntyai nveva rasasyo caiva sarvatvāyedaṃ hi yadā varṣatyathauṣadhayo jāyanta 'oṣadhīrjagdhvāpaḥ pītvā tata eṣa rasaḥ sambhavati tasmādu rasasyo caiva sarvatvāya tasmādyadyenaṃ kṣīraṃ kevalam pāne 'bhyābhavedudastokamāścotayitavai brūyācāntyai nveva rasasyo caiva sarvatvāya

He illumines it (with a burning straw) in order that he may know when it is done. He then pours some water to it (with the sruva), both for the sake of appeasement, and in order to supplement the juice. For when it rains here; then plants spring up; and in consequence of the plants being eaten and the water drunk, this juice is produced: hence it is in order to supplement the juice (that he pours water to it); and therefore, if it should happen to him to have to drink pure milk, let him have one drop of water poured into it, both for the sake of appeasement, and in order to supplement the juice.