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 Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
नैष त्वया मनुष्येन्द्र वधमर्हति सर्पराट् । अनेन पीतममृतमथ वा अजरामरः
naiṣa tvayā manuṣyendra vadhamarhati sarparāṭ | anena pītamamṛtamatha vā ajarāmaraḥ
SB 12.6.24