One verse

Srimad Bhagavatam 2.7.13

2.7.13

क्षीरोदधावमरदानवयूथपाना- मुन्मथ्नताममृतलब्धय आदिदेव: । पृष्ठेन कच्छपवपुर्विदधार गोत्रं निद्राक्षणोऽद्रिपरिवर्तकषाणकण्डू: ॥ १३ ॥

ksirodadhav amara-danava-yuthapanam unmathnatam amrta-labdhaya adi-devah prsthena kacchapa-vapur vidadhara gotram nidraksano ’dri-parivarta-kasana-kanduh

ksira
milk
udadhau
in the ocean of
amara
the demigods
danava
the demons
yutha-panam
of the leaders of both hosts
unmathnatam
while churning
amrta
nectar
labdhaya
for gaining
adi-devah
the primeval Lord
prsthena
by the backbone
kacchapa
tortoise
vapuh
body
vidadhara
assumed
gotram
the Mandara Hill
nidraksanah
while partly sleeping
adri-parivarta
rolling the hill
kasana
scratching
kanduh
itching

The primeval Lord then assumed the tortoise incarnation in order to serve as a resting place (pivot) for the Mandara Mountain, which was acting as a churning rod. The demigods and demons were churning the ocean of milk with the Mandara Mountain in order to extract nectar. The mountain moved back and forth, scratching the back of Lord Tortoise, who, while partially sleeping, was experiencing an itching sensation.