Shatapatha Brahmana 5.3.1.8

5.3.1.8

atha śvo bhūte | saṃgrahīturgṛhānparetyāśvinaṃ dvikapālam puroḍāśaṃ nirvapati sayonī vā aśvinau sayonī savyaṣṭhṛsārathī samānaṃ hi rathamadhitiṣṭhatastasmādāśvino bhavatyetadvā asyaikaṃ ratnaṃ yatsaṃgrahītā tasmā evaitena sūyate taṃ svamanapakramiṇaṃ kurute tasya yamau gāvau dakṣiṇā tau hi sayonī yadyamau yadi yamau na vindedapyanūcīnagarbhāveva gāvau dakṣiṇā syātāṃ tā u hyapi samānayonī

And on the following day he goes to the house of the Charioteer (samgrahîtri), and prepares a cake on two potsherds for the Asvins; for the two Asvins are of the same womb; and so are the chariot fighter and the driver (sârathi) of the same womb (standing-place), since they stand on one and the same chariot: hence it is for the Asvins. And he, the charioteer, 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 (jewel) is a pair of twin bullocks, for such twin bullocks are of the same womb. If he cannot obtain twins, two bullocks produced by successive births (of the same cow) may also form the sacrificial fee, for such also are of the same womb.