Śatapatha Brāhmaṇa ("of a hundred paths") 13.4.2.15

13.4.2.15

etasyāṃ saṃsthitāyām upotthāyādhvaryuśca yajamānaścāśvasya dakṣiṇe karṇa ājapato vibhūrmātrā prabhūḥ pitreti tasyoktam brāhmaṇamathainamudañcam prāñcam prasṛjata eṣā hobhayeṣāṃ devamanuṣyāṇāṃ digyadudīcī prācī svāyāmevainaṃ taddiṣi dhatto na vai sva āyatane pratiṣṭhito riṣyatyariṣṭyai

When this (offering) is completed, the Adhvaryu and the Sacrificer rise, and whisper in the horse's right ear (Vâg. S. XXII, 19), 'Plenteous by thy mother, strengthful by dry father...!' the meaning of this has been explained. They then set it free towards the north-east, for that--to wit, the north-east--is the region of both gods and men: they thus consign it to its own region, in order to its suffering no injury, for one who is established in his own home suffers no injury.