Śatapatha Brāhmaṇa ("of a hundred paths") 2.6.1.6

2.6.1.6

atha pitṛbhyo 'gniṣvāttebhyaḥ | nivānyāyai dugdhe sakṛdupamathita ekaśalākayā mantho bhavati sakṛdu hyeva parāñcaḥ pitarastasmātsakṛdupamathito bhavatyetāni havīṃṣi bhavanti

Then a porridge is (prepared) for the Pitaro ’gnishvâttâh (by the ground half of the parched grain) being mixed with the milk of a cow suckling an adopted calf, by stirring it once with a single splinter. It is indeed once for all that the fathers have departed, and hence is stirred but once. These are the oblations.