Once upon a time he [Vyāsadeva], as the sun rose, took his morning ablution in the waters of the Sarasvatī and sat alone to concentrate.
Srimad Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
स कदाचित्सरस्वत्या उपस्पृश्य जलं शुचिः । विविक्तदेश आसीन उदिते रविमण्डले
sa kadācitsarasvatyā upaspṛśya jalaṃ śuciḥ | viviktadeśa āsīna udite ravimaṇḍale
SB 1.4.15