Śatapatha Brāhmaṇa ("of a hundred paths") 13.3.5.4

13.3.5.4

etāṃ ha vai muṇḍibha audanyaḥ brahmahatyāyai prāyaścittiṃ vidāṃ cakāra yadbrahmaihatyāyā āhutiṃ juhoti mṛtyumevāhutyā tarpayitvā paripāṇaṃ kṛtvā brahmaghne bheṣajaṃ karoti tasmādyasyaiṣāśvamedha āhutirhūyate'pi yo'syāparīṣu prajāyām brāhmaiṇaṃ hanti tasmai bheṣajaṃ karoti

Mundibha Audanya it was who discovered this atonement for the slaying of a Brahman; and when one offers the oblation to the Brahmahatyâ he prepares a remedy for the slayer of a Brahman by satisfying Death himself with an oblation, and making a protection for him (the slayer). At whosoever's Asvamedha, therefore, this oblation is offered, even if in after-times any one in his family kills a Brahman, he thereby prepares a remedy (expiation) for him.