Śatapatha Brāhmaṇa ("of a hundred paths") 10.6.5.7

10.6.5.7

so'kāmayata medhyaṃ ma idaṃ syādātmanvyanena syāmiti tato'śvaḥ samabhavadyadaśvattanmedhyamabhūditi tadevāśvamedhasyāśvamedhatvameṣa ha vā aśvamedhaṃ veda ya enamevaṃ veda

He desired, 'May this (body) of mine be sacrificially pure: may I thereby be possessed of a self!' Thereupon the horse (asva) was produced; and because that which was swelling (asvat) became pure (medhya) therefore the name Asvamedha (belongs to that sacrifice). He, indeed, knows the Asvamedha who thus knows him.