SB 12.6.50
rg-atharva-yajuh-samnam
rasir uddhrtya vargasah
catasrah samhitas cakre
mantrair mani-gana iva
Translation:
Srila Vyasadeva separated the mantras of the Rg, Atharva, Yajur and Sama Vedas into four divisions, just as one sorts out a mixed collection of jewels into piles. Thus he composed four distinct Vedic literatures.
Purport:
When Lord Brahma first spoke the four Vedas with his four mouths, the mantras were mixed together like an unsorted collection of various types of jewels. Srila Vyasadeva sorted the Vedic mantras into four divisions (samhitas), which thus became the recognizable Rg, Atharva, Yajur and Sama Vedas.