Śatapatha Brāhmaṇa ("of a hundred paths") 13.2.3.1

13.2.3.1

devā vā aśvamedhe pavamānaṃ svargaṃ lokaṃ na prājānaṃstamaśvaḥ prājānādyadaśvamedhe'śvena pavamānāya sarpanti svargasya lokasya prajñātyai pucamanvārabhante svargasyaiva lokasya samaṣṭyai na vai manuṣyaḥ svargaṃ lokamañjasā vedāśvo vai svargaṃ lokamañjasā veda

Now, the gods did not know the Pavamâna at the Asvamedha to be the heavenly world, but the horse knew it. When, at the Asvamedha, they glide along with the horse for the Pavamâna (-stotra), it is for getting to know (the way to) the heavenly world; and they hold on to the horse's tail, in order to reach the heavenly world; for man does not rightly know (the way to) the heavenly world, but the horse does rightly know it