Śatapatha Brāhmaṇa ("of a hundred paths") 5.3.1.7

5.3.1.7

atha śvo bhūte | kṣatturgṛhānparetya sāvitraṃ dvādaśakapālaṃ vāṣṭākapālaṃ vā puroḍāśaṃ nirvapati savitā vai devānām prasavitā prasavitā vai kṣattā tasmātsāvitro bhavatyetadvā asyaikaṃ ratnaṃ yatkṣattā tasmā evaitena sūyate taṃ svamanapakramiṇaṃ kurute tasya śyeto 'naḍvāndakṣiṇaiṣa vai savitā ya eṣa tapatyeti vā eṣa etyanaḍvānyuktastadyacyeto bhavati śyeta iva hyeṣa udyaṃścāstaṃ ca yanbhavati tasmācyeto 'naḍvāndakṣiṇā

And on the following day he goes to the house of the Chamberlain (kshattri), and prepares a cake on either twelve, or eight, potsherds for Savitri; for Savitri is the impeller (prasavitri) of the gods, and the chamberlain is an impeller: hence it is for Savitri. And he, the chamberlain, assuredly is one of his (the king's) jewels: it is for him that he thereby is consecrated, and him he makes his own faithful (follower). The sacrificial fee for this (jewel) is a reddish-white draught-bullock; for Savitri is he that burns yonder, and he (the sun) indeed moves along; and the draught-bullock also moves along, when yoked. And as to why it is a reddish-white one;--reddish-white indeed is he (the sun) both in rising and in setting: therefore the sacrificial fee is a reddish-white draught-bullock.