Śatapatha Brāhmaṇa ("of a hundred paths") 3.5.1.19

3.5.1.19

athaibhyaḥ sūryaṃ dakṣiṇāmānayan | tam pratyagṛhṇaṃstasmādu ha smāhuraṅgiraso vayaṃ vā ārtvijīnāḥ smo vayaṃ dakṣiṇīyā api vā asmābhireṣa pratigṛhīto ya eṣa tapatīti tasmātsadyaḥkriyo 'śvaḥ śveto dakṣiṇā

Thereupon they brought Sûrya (the sun) to them, and they accepted him. Wherefore the Aṅgiras say, 'Verily, we are fit for the sacrificial office, we are worthy to receive Dakshinâs; yea, even he that burns yonder has been received by us!' Hence a white horse is the sacrificial fee for the Sadyahkrî.