Śatapatha Brāhmaṇa ("of a hundred paths") 5.3.1.2

5.3.1.2

atha śvo bhūte | purohitasya gṛhānparetya bārhaspatyaṃ caruṃ nirvapati bṛhaspatirvai devānāṃ purohita eṣa vā etasya purohito bhavati tasmādbārhaspatyo bhavatyetadvā asyaikaṃ ratnaṃ yatpurohitastasmā evaitena sūyate taṃ svamanapakramiṇaṃ kurute tasya śitipṛṣṭho gaurdakṣiṇaiṣā vā ūrdhvā bṛhaspaterdiktadeṣa upariṣṭādaryamṇaḥ panthāstasmācitipṛṣṭho bārhaspatyasya dakṣiṇā

And on the following day, he goes to the house of the Purohita (the king's court chaplain), and prepares a pap for Brihaspati; for Brihaspati is the Purohita of the gods, and that (court chaplain) is the Purohita ('praepositus') of that (king): hence it is for Brihaspati. And he, the Purohita, assuredly is one of his (the king's) jewels: it is for him that he is thereby consecrated, and him he makes his own faithful follower. The sacrificial fee for this is a white-backed bullock; for to Brihaspati belongs that upper region, and there above lies that path of Aryaman (the sun): therefore the fee for the Bârhaspatya (oblation) is a white-backed (bullock).