Atharvavedaparishishta 2.5.4

2.5.4

trayo lokās trayo devās trayo vedās trayo 'gnayaḥ | ardhamātre layaṃ yānti vedaś cātharvaṇaḥ smṛtaḥ ||

The three worlds, the three gods, the three Vedas, and the three fires / merge into the half-mora, and that is traditionally known as the Atharvaveda.