Shiva Purana
एवमाद्यैर्नरः पापैरुत्क्रांतिसमनंतरम्॥ शरीरयातनार्थाय सर्वाकारमवाप्नुयात् ॥ ५२ ॥
By these sins the man after his death is born as a serpent for suffering the tortures.
english translation
evamAdyairnaraH pApairutkrAMtisamanaMtaram॥ zarIrayAtanArthAya sarvAkAramavApnuyAt ॥ 52 ॥
hk transliteration by Sanscript