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.
Srimad Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
क्षीरोदधावमरदानवयूथपाना- मुन्मथ्नताममृतलब्धय आदिदेवः । पृष्ठेन कच्छपवपुर्विदधार गोत्रं निद्राक्षणोऽद्रिपरिवर्तकषाणकण्डूः
kṣīrodadhāvamaradānavayūthapānā- munmathnatāmamṛtalabdhaya ādidevaḥ | pṛṣṭhena kacchapavapurvidadhāra gotraṃ nidrākṣaṇo'driparivartakaṣāṇakaṇḍūḥ
SB 2.7.13