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.