Skandapurana 5.266.15

5.266.15

yugmāṃśca ṛtvijaḥ kāryāścaturdikṣu yathākramam..
bahvṛco'dhvaryaścaiva cchaṃdogātharvaṇāvapi.. 15..

And twin officiating priests should be appointed in the four directions in proper order: / a Ṛgvedic priest, a Yajurvedic priest, a Sāmavedic priest, and an Atharvavedic priest.