Śatapatha Brāhmaṇa ("of a hundred paths") 3.2.3.20

3.2.3.20

śiro vai yajñasyātithyam | bāhū prāyaṇīyodayanīyāvabhito vai śiro bāhū bhavatastasmādabhita ātithyamete haviṣī bhavataḥ prāyaṇīyaścodayanīyaśca

The hospitable reception (of King Soma) verily is the head of the sacrifice, and the opening and closing oblations are its arms. But the arms are on both sides of the head, and hence those two oblations, the Prâyanîya and Udayanîya, are made on both sides of (before and after) the reception.