Srimad Bhagavatam Canto 06, Chapter 09, Text 18

SB 6.9.18

yenavrta ime lokas
 tapasa tvastra-murtina
sa vai vrtra iti proktah
 papah parama-darunah
 
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada: 
 
That very fearful demon, who was actually the son of Tvasta, covered all the planetary systems by dint of austerity. Therefore he was named Vrtra, or one who covers everything.
 
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada: 
 
In the Vedas it is said, sa imaḻ lokan avrnot tad vrtrasya vrtratvam: because the demon covered all the planetary systems, his name was Vrtrasura.
Srimad Bhagavatam Canto 06, Chapter 09, Text 13-17
Srimad Bhagavatam Canto 06, Chapter 09, Text 19