Atharvavedaparishishta 2.4.2

2.4.2

ātharvaṇād ṛte nānyo niyojyo 'tharvavid guruḥ | nṛpeṇa jayakāmena nirmito 'gnir ivādhvare ||

Except for an Atharvan, no other should be appointed as spiritual guide who knows the Atharvan / by a king desiring victory, just as fire is prepared at a sacrificial session.