Srimad-Bhagavatam: Canto 10 - Chapter 1 - Verse 47
Sanskrit:
अवध्योऽयं ममाप्येष वैरोचनिसुतोऽसुर: ।प्रह्रादाय वरो दत्तो न वध्यो मे तवान्वय: ॥ ४७ ॥
ITRANS:
avadhyo ’yaṁ mamāpy eṣavairocani-suto ’suraḥprahrādāya varo dattona vadhyo me tavānvayaḥ
Translation:
I will not kill this demonic son of Vairocani, for I gave Prahlāda Mahārāja the benediction that I would not kill any of his descendants.