Śatapatha Brāhmaṇa ("of a hundred paths") 6.4.4.13

6.4.4.13

tadyadito yatām | aśvaḥ prathama eti tasmākṣatriyam prathamaṃ yantamitare trayo varṇāḥ paścādanuyantyatha yadamuta āyatāmajaḥ prathama eti tasmādbrāhmaṇam prathamaṃ yantamitare trayo varṇāḥ paścādanuyantyatha yannaiveto yatāṃ nāmuto rāsabhaḥ prathama eti tasmānna kadā cana brāhmaṇaśca kṣatriyā vaiśyaṃ ca śūdraṃ ca paścādanvitastasmādevaṃ yantyapāpavasyasāyātho brahmaṇā caivaitatkṣatreṇa caitau varṇāvabhitaḥ parigṛhṇīte 'napagṛhṇīte kurute

And inasmuch as, in going from here, the horse goes first, therefore the Kshatriya, going first, is followed by the three other castes; and inasmuch as, in returning from there, the he-goat goes first, therefore the Brâhmana, going first, is followed by the three other castes. And inasmuch as the ass does not go first, either in going from here, or in coming back from there, therefore the Brâhmana and Kshatriya never go behind the Vaisya and Sûdra hence they walk thus in order to avoid a confusion between good and bad. And, moreover, he thus encloses those two castes (the Vaisya and Sûdra) on both sides by the priesthood and the nobility, and makes them submissive.