Śatapatha Brāhmaṇa ("of a hundred paths") 3.4.1.1

3.4.1.1

śiro vai yajñasyātithyaṃ bāhū prāyaṇīyodayanīyau | abhito vai śiro bāhū bhavatastasmādabhita ātithyamete haviṣī bhavataḥ prāyaṇīyaścodayanīyaśca

Verily, the guest-offering is the head of the sacrifice, and the Prâyanîya and Udayanîya are its arms. But the arms are on both sides of the head: therefore those two oblations, the Prâyanîya and Udayanîya, are on both sides of the guest-offering.