"Know that as from a fire laid with green wood, when kindled, clouds of
smoke separately issue forth, so, verily, from this great being has been
breathed forth that which is the Rig Veda, the Yajur Veda, the Sama
Veda, the Atharvans and the Angirasas, legend (itihasa), ancient lore
(purana), sciences (vidya), mystic doctrines (upanishad), verses
(sloka), aphorisms (sutra), explanations (anuvyakhyana) and commentaries
(vyakhyana)."
Krishna Yajur Veda, Maitri Upanishad 6.32