Śatapatha Brāhmaṇa ("of a hundred paths") 7.3.2.16

7.3.2.16

sa śuklaḥ syāt | taddhyetasya rūpaṃ ya eṣa tapati yadi śuklaṃ na vindedapyaśuklaḥ syādaśvastveva syādyadyaśvaṃ na vindedapyanaḍvāneva syādāgneyo vā anaḍvānagniru sarveṣām pāpmanāmapahantā

It should be a white (horse), for that is a form of him (the sun) who burns yonder. If he cannot obtain a white one, one that is not white might do; but a horse it should be. If he cannot obtain a horse, even an ox might do, for the ox is of Agni's nature, and Agni is the repeller of all evils.