Srimad-Bhagavatam: Canto 6 - Chapter 1 - Verse 18

Sanskrit:

येनावृता इमे लोकास्तपसा त्वाष्ट्रमूर्तिना ।स वै वृत्र इति प्रोक्त: पाप: परमदारुण: ॥ १८ ॥

ITRANS:

yenāvṛtā ime lokāstapasā tvāṣṭra-mūrtināsa vai vṛtra iti proktaḥpāpaḥ parama-dāruṇaḥ

Translation:

That very fearful demon, who was actually the son of Tvaṣṭā, covered all the planetary systems by dint of austerity. Therefore he was named Vṛtra, or one who covers everything.

Purport:

In the Vedas it is said, sa imāḻ lokān āvṛṇot tad vṛtrasya vṛtratvam: because the demon covered all the planetary systems, his name was Vṛtrāsura.