Srimad Bhagavatam Canto 12, Chapter 06, Text 24

SB 12.6.24

naisa tvaya manusyendra
 vadham arhati sarpa-rat
anena pitam amrtam
 atha va ajaramarah
 
Translation: 
 
O King among men, it is not fitting that this king of snakes meet death at your hands, for he has drunk the nectar of the immortal demigods. Consequently he is not subject to the ordinary symptoms of old age and death.
Srimad Bhagavatam Canto 12, Chapter 06, Text 23
Srimad Bhagavatam Canto 12, Chapter 06, Text 25