Śatapatha Brāhmaṇa ("of a hundred paths") 13.3.3.6

13.3.3.6

saṃkṛtyacāvākasāma bhavati utsannayajña iva vā eṣa yadaśvamedhaḥ kiṃ vā hyetasya kriyate kiṃ vā na yatsaṃkṛtyacāvākasāma bhavatyaśvasyaiva sarvatvāya sarvastomo'tirātra uttamamaharbhavati sarvaṃ vai sarvastomo'tirātraḥ sarvamaśvamedhaḥ sarvasyāptyai sarvasyāvaruddhyai

The Samkriti is the Akhâvâka's Sâman;-- that Asvamedha, indeed, is, as it were, a disused sacrifice, for what is performed thereof, and what is not? When the Samkriti is the Akhâvâka's Sâman, it is for (bringing about) the completeness of the horse (sacrifice). The last day is an Atirâtra with all the (six) Stomas, in order to his (the Sacrificer's) obtaining everything, for an Atirâtra with all the Stomas is everything, and the Asvamedha is everything.