Śatapatha Brāhmaṇa ("of a hundred paths") 13.2.3

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

Were the Udgâtri to chant the Udgîtha, it would be even as if one who does not know the country were to lead by another (than the right) way. But if, setting aside the Udgâtri, he chooses the horse for (performing) the Udgîtha, it is just as when one who knows the country leads on the right way: the horse leads the Sacrificer rightly to the heavenly world. It makes 'Hiṅ,' and thereby makes the. Sâman itself to be 'hiṅ': this is the Udgîtha. They pen up mares, (and on seeing the horse) they utter a shrill sound: as when the chanters sing, such like is this. The priests’ fee is gold weighing a hundred (grains): the mystic import of this has been explained.