Srimad-Bhagavatam: Canto 12 - Chapter 1 - Verse 50

Sanskrit:

ऋगथर्वयजु:साम्नां राशीरुद्‍धृत्य वर्गश: ।चतस्र: संहिताश्चक्रे मन्त्रैर्मणिगणा इव ॥ ५० ॥

ITRANS:

ṛg-atharva-yajuḥ-sāmnāṁrāśīr uddhṛtya vargaśaḥcatasraḥ saṁhitāś cakremantrair maṇi-gaṇā iva

Translation:

Śrīla Vyāsadeva separated the mantras of the Ṛg, Atharva, Yajur and Sāma 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 Brahmā first spoke the four Vedas with his four mouths, the mantras were mixed together like an unsorted collection of various types of jewels. Śrīla Vyāsadeva sorted the Vedic mantras into four divisions (saṁhitās), which thus became the recognizable Ṛg, Atharva, Yajur and Sāma Vedas.