Śatapatha Brāhmaṇa ("of a hundred paths") 13.3.3.1

13.3.3.1

yattisro'nuṣṭubho bhavanti tasmādaśvastribhistiṣṭhaṃstiṣṭhati yaccatasro gāyatryastasmādaśvaḥ sarvaiḥ padbhiḥ pratidadhatpalāyate paramaṃ vā etaccando yadanuṣṭupparamo'śvaḥ paśūnām paramaścatuṣṭoma stomānām parameṇaivainam paramatāṃ gamayati

Inasmuch as there are three Anushtubh verses (on the first day), therefore the horse, when standing, stands on three (feet); and inasmuch as (they are made into) four Gâyatrî verses, therefore the horse, when stepping out, scampers off on all (four) feet. For that Anushtubh, doubtless, is the highest metre, and the horse is the highest of animals; and the Katushtoma is the highest of Stomas: by means of what is highest he thus causes him (the Sacrificer) to reach the highest position.