Srimad Bhagavatam Canto 10, Chapter 63, Text 47

SB 10.63.47

avadhyo ’yam mamapy esa
 vairocani-suto ’surah
prahradaya varo datto
 na vadhyo me tavanvayah
 
Translation: 
 
I will not kill this demonic son of Vairocani, for I gave Prahlada Maharaja the benediction that I would not kill any of his descendants.
Srimad Bhagavatam Canto 10, Chapter 63, Text 46
Srimad Bhagavatam Canto 10, Chapter 63, Text 48