Shatapatha Brahmana 13.2.8.1

13.2.8.1

devā vā udañcaḥ svargaṃ lokaṃ na prājānaṃstamaśvaḥ prājānādyadaśvenodañco yanti svargasya lokasya prajñātyai vāso'dhivāsaṃ hiraṇyamityaśvāyopastṛṇanti yathā nānyasmai paśave tasminnenamadhi saṃjñapayantyanyairevainaṃ tatpaśubhirvyākurvanti

Now the gods, when going upwards, did not know (the way to) the heavenly world, but the horse knew it: when they go upwards with the horse, it is in order to know (the way to) the heavenly world. 'A cloth, an upper cloth, and gold,' this is what they spread out for the horse: thereon they quiet (slay) it, as (is done) for no other victim; and thus they separate it from the other victims.