Srimad-Bhagavatam: Canto 1 - Chapter 1 - Verse 16
सुरासुराणामुदधिं मथ्नतां मन्दराचलम् ।दध्रे कमठरूपेण पृष्ठ एकादशे विभु: ॥ १६ ॥
surāsurāṇām udadhiṁmathnatāṁ mandarācalamdadhre kamaṭha-rūpeṇapṛṣṭha ekādaśe vibhuḥ
The eleventh incarnation of the Lord took the form of a tortoise whose shell served as a pivot for the Mandarācala Hill, which was being used as a churning rod by the theists and atheists of the universe.
Once both the atheists and the theists were engaged in producing nectar from the sea so that all of them could become deathless by drinking it. At that time the Mandarācala Hill was used as the churning rod, and the shell of Lord Tortoise, the incarnation of Godhead, became the resting place (pivot) of the hill in the seawater.