Shatapatha Brahmana 13.3.1.1

13.3.1.1

prajāpaterakṣyaśvayat tatparāpatattato'śvaḥ samabhavadyadaśvayattadaśvasyāśvatvaṃ taddevā aśvamedhenaiva pratyadadhureṣa ha vai prajāpatiṃ sarvaṃ karoti yo'śvamedhena yajate sarva eva bhavati sarvasya vā eṣā prāyaścittiḥ sarvasya bheṣajaṃ sarvaṃ vā etena pāpmānaṃ devā atarannapi vā etena brahmahatyāmataraṃstarati sarvam pāpmānaṃ tarati brahmahatyāṃ yo'śvamedhena yajate

Pragâpati's eye swelled; it fell out: thence the horse was produced; and inasmuch as it swelled (asvayat), that is the origin and nature of the horse (asva). By means of the Asvamedha the gods restored it to its place; and verily he who performs the Asvamedha makes Pragâpati complete, and he (himself) becomes complete; and this, indeed, is the atonement for everything, the remedy for everything. Thereby the gods redeem all sin, yea, even the slaying of a Brahman they thereby redeem; and he who performs the Asvamedha redeems all sin, he redeems the slaying of a Brahman.