Śatapatha Brāhmaṇa ("of a hundred paths") 5.2.5.8

5.2.5.8

atha yatpauṣṇaḥ | carurbhavati yānevāsmā agnirdātā paśūndadāti teṣvevaitadantataḥ pratitiṣṭhati yadvai paśumānkarma cikīrṣati śaknoti vai tatkartuṃ tatpaśūnevaitadupaiti paśumāntsūyā iti tasya śyāmo gaurdakṣiṇā sa hi pauṣṇo yacyāmo dve vai śyāmasya rūpe śuklaṃ caiva loma kṛṣṇaṃ ca dvandvaṃ vai mithunam prajananaṃ vai pūṣā paśavo hi pūṣā paśavo hi prajananam mithunamevaitatprajananaṃ kriyate tasmācyāmo gaurdakṣiṇā

And why there is a pap for Pûshan;--whatever cattle Agni, the giver, gives him, therewith he thereby finally establishes him, and whatever work he wishes to do with his cattle, that he is able to do: thus he comes by cattle, thinking, 'May I be consecrated, possessed of cattle!' A dark-grey bull is the fee for this (offering), for such a dark-grey one is of Pûshan's nature: there are two forms of the dark-grey, the white hair and the black; and 'two and two' means a productive pair, and Pûshan represents productiveness, for Pûshan is cattle, and cattle means productiveness: thus a productive pair is obtained,--hence a dark-grey bull is the sacrificial fee.