Śatapatha Brāhmaṇa ("of a hundred paths") 13.4.2.1

13.4.2.1

etasyāṃ tāyamānāyām aśvaṃ niktvodānayanti yasmintsarvāṇi rūpāṇi bhavanti yo vā javasamṛddhaḥ sahasrārham pūrvyaṃ yo dakṣiṇāyāṃ dhuryapratidhuraḥ

Whilst this (offering to Pûshan) is being performed, the horse, having been cleansed, is led up--being one which is marked with all colours, or which is perfect in speed, worth a thousand (cows), in its prime, and without its match under the right-side yoke.