Shatapatha Brahmana 4.3.2.2

4.3.2.2

taṃ vai prāñcamāsīnamāhvayate | sarve vā anya udgātuḥ prāñca ārtvijyaṃ kurvanti tatho hāsyaitatprāgevārtvijyaṃ kṛtam bhavati

[The Hotri] calls upon that (Adhvaryu) seated (before him) with his face towards the east. For all others except the Udgâtri perform their priestly duties while facing the east, and in this manner that priestly duty of his is performed towards the east.